Float glass for chemical strengthening

ABSTRACT

The present invention relates to a float glass for chemical strengthening, containing a bottom surface coming into contact with a molten metal at the time of forming and a top surface opposing the bottom surface, in which a difference Δ(N—Na 2 O 2 ) determined by subtracting a square of a normalized Na 2 O surface concentration of the bottom surface which is a value obtained by dividing an Na 2 O concentration in the bottom surface by an Na 2 O concentration at a depth position of 100 μM therefrom, from a square of a normalized Na 2 O surface concentration of the top surface which is a value obtained by dividing an Na 2 O concentration in the top surface by an Na 2 O concentration at a depth position of 100 μm therefrom, is 0.040 or less.

TECHNICAL FIELD

The present invention relates to a float glass for chemicalstrengthening.

BACKGROUND ART

Recently, in a flat panel display device such as mobile phone orpersonal digital assistant (PDA), a thin sheet-like cover glass isdisposed on a front surface of a display to extend in a wider regionthan the image display area with an aim to enhance the protection andbeauty of the display.

Such the flat panel display device is required to be lightweight andthin and in order to meet this requirement, the thickness of a coverglass used for display protection is also required to be reduced.

However, decreasing the thickness of the cover glass causes the problemsthat the strength is reduced and the cover glass itself may be broken bydropping, etc. during use or carrying and therefore its primary role ofprotecting the display device cannot be fulfilled.

Accordingly, in the conventional cover glass, with an aim to improve thescratch resistance, a compressive stress layer is formed on the surfaceby chemically strengthening a float glass produced by a float method,and the scratch resistance of the cover glass is thereby enhanced.

It has been reported that warpage occurs in a float glass after chemicalstrengthening and impairs the flatness (Patent Document 1). The warpageis thought to occur due to the difference in the chemical strengtheningbehavior between a glass surface (hereinafter, also referred to as topsurface) that is out of contact with molten tin at the time of floatforming, and a glass surface (hereinafter, also referred to as bottomsurface) that comes into contact with molten tin.

Heretofore, the reason for the difference in the chemical strengtheningbehaviors between the top surface of the float glass and the bottomsurface has been considered to be the invasion of a molten metal intothe glass surface in contact with the molten metal at the time of floatforming (Patent Document 1).

In Patent Document 1, it is disclosed that the warpage is improved whena sheet-like material produced by a float method and processed is,without applying surface polishing, dipped in or put into contact withLi ion, Na ion or a mixed inorganic salt thereof and then chemicallystrengthened.

Furthermore, in order to reduce the warpage, conventionally employed isa coping method of decreasing the strengthening stress produced bychemical strengthening or performing chemical strengthening aftersubjecting the top surface and the bottom surface of a float glass to agrinding treatment, a polishing treatment, etc. so as to remove asurface heterogeneous layer.

PRIOR ART DOCUMENT Patent Document

Patent Document 1: Japanese Patent No. 2,033,034

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

However, in the method described in Patent Document 1, the float glassneeds to be dipped in a mixed inorganic salt before chemicalstrengthening, and this is cumbersome. In addition, the method ofdecreasing the strengthening stress may afford insufficient strength tothe float glass after chemical strengthening.

Furthermore, the method of subjecting the top surface and the bottomsurface of a float glass to a grinding treatment, a polishing treatment,etc. before chemical strengthening has a problem from the standpoint ofenhancing the productivity, and it is preferable to omit such thegrinding treatment, the polishing treatment, etc.

Accordingly, an object of the present invention is to provide a floatglass for chemical strengthening, which can effectively suppress thewarpage after chemical strengthening.

Means for Solving the Problems

The present inventors have found that the main cause of warpageoccurring due to the difference in the chemical strengthening behaviorsbetween the bottom surface and the top surface when chemicallystrengthening a soda-lime glass produced by a float process is notnecessarily a metal invading a glass surface in contact with the moltenmetal at the time of float forming but is the difference in theweathering degree between the top surface and the bottom surface,namely, the difference in the degree of hydration-dealkalization.

Furthermore, they found that by suppressing the effect thereof, thestrengthening degree in chemical strengthening can be equalized betweenthe top surface and the bottom surface and therefore the warpage of afloat glass after chemical strengthening can be reduced. In addition,they found that the weathering has a profound effect particularly in alow DOL region where the depth of compressive stress layer (DOL) istypically 20 μm or less, 15 μm or less or 10 μm or less, and bydiminishing the effect of weathering degree in this region, the warpageof a float glass after chemical strengthening can be effectivelyreduced. They have accomplished the present invention based on thesefindings.

The present invention is as the following.

1. A float glass for chemical strengthening, containing a bottom surfacecoming into contact with a molten metal at the time of forming and a topsurface opposing the bottom surface, in which a difference Δ(N—Na₂O²)determined by subtracting a square of a normalized Na₂O surfaceconcentration of the bottom surface which is a value obtained bydividing an Na₂O concentration in the bottom surface by an Na₂Oconcentration at a depth position of 100 μm therefrom, from a square ofa normalized Na₂O surface concentration of the top surface which is avalue obtained by dividing an Na₂O concentration in the top surface byan Na₂O concentration at a depth position of 100 μm therefrom, is 0.040or less,

where each Na₂O concentration is a value measured by a fluorescent X-rayanalysis using an Na-Kα ray.

A float glass for chemical strengthening, containing a bottom surfacecoming into contact with a molten metal at the time of forming and a topsurface opposing the bottom surface, in which a Δion exchange amount 1which is a value obtained by subtracting an ion exchange amount 1 in thebottom surface from an ion exchange amount 1 in the top surface is 0.32or less,

where the ion exchange amount 1 is a value determined according to thefollowing formula (2-1):

Ion exchange amount 1=5.51×(normalized Na₂O surfaceconcentration)−0.038×(Sn concentration)  formula (2-1)

in formula (2-1),

the normalized Na₂O surface concentration is a value obtained bydividing an Na₂O concentration at the surface by an Na₂O concentrationat a depth position of 100 μm, where the Na₂O concentration is a valuemeasured by a fluorescent X-ray analysis using an Na-Kα ray; and

the Sn concentration is an Sn deposition amount (unit: as SnO₂μg/cm²)per unit area of the top surface and the bottom surface. In the presentspecification, the unit of the Sn deposition amount per unit area isindicated as “as SnO₂μg/cm²” in order to specify that the Sn depositionamount per unit area is expressed by a deposition mass in terms of SnO₂per 1 cm² when Sn is assumed to exist in the form of SnO₂. In thepresent specification, the Sn deposition amount (unit: μg/cm²) per unitarea possesses the same meaning as the Sn deposition amount (unit: asSnO₂μg/cm²) per unit area.

3. A float glass for chemical strengthening, containing a bottom surfacecoming into contact with a molten metal at the time of forming and a topsurface opposing the bottom surface, in which W1 determined according tothe following formula (3-1) is 56 or less:

W1=−16×(ΔH/Si)−6.47×(Sn concentration difference)−43.8×(Δion exchangeamount 1)  formula (3-1)

in formula (3-1),

the ΔH/Si is a value obtained by subtracting a normalized hydrogenconcentration in the bottom surface from a normalized hydrogenconcentration in the top surface, where the normalized hydrogenconcentration is a value obtained by dividing an average hydrogenconcentration at a depth of 0 to 10 μm by an average hydrogenconcentration at a depth of 105 to 110 μm, in which the average hydrogenconcentration at a depth of 0 to 10 μm and the average hydrogenconcentration at a depth of 105 to 110 μm are values measured under thefollowing analysis conditions:

(Analysis Conditions)

Measurement apparatus: secondary ion mass spectrometry apparatus with aquadrupole mass spectrometer

Primary ion species: Cs⁺

Primary accelerating voltage: 5.0 kV

Primary ion current: 1 μA

Primary ion incident angle (angle from direction perpendicular to samplesurface): 60°

Luster size: 200×200 μm²

Detection region: 40×40 μm²

Secondary ion polarity: minus

Electron gun for neutralization: used;

in formula (3-1),

the Sn concentration difference is a difference obtained by subtractingan Sn deposition amount (unit: μg/cm²) per unit area of the top surfacefrom an Sn deposition amount (unit: as SnO₂μg/cm²) per unit area of thebottom surface, and in the case where the glass does not contain SnO₂,this is equivalent to the Sn deposition amount per unit area of thebottom surface;

andin formula (3-1),

the Δion exchange amount 1 is a value obtained by subtracting an ionexchange amount 1 in the bottom surface from an ion exchange amount 1 inthe top surface, where the ion exchange amount 1 is determined accordingto the following formula:

Ion exchange amount 1=5.51×(normalized Na₂O surfaceconcentration)−0.038×(Sn concentration)

in which

the normalized Na₂O surface concentration is a value obtained bydividing an Na₂O concentration at the surface by an Na₂O concentrationat a depth position of 100 where the Na₂O concentration is a valuemeasured by a fluorescent X-ray analysis using an Na-Kα ray.

4. A float glass for chemical strengthening, containing a bottom surfacecoming into contact with a molten metal at the time of forming and a topsurface opposing the bottom surface, in which an absolute value of W2determined according to the following formula (4-1) is 56 or less:

W2=9.18×Δ[(ion exchange amount)/(H/Si)]+49  formula (4-1)

in formula (4-1),

the Δ[(ion exchange amount)/(H/Si)] is a value determined by subtractinga value obtained by dividing an ion exchange amount 1 in the bottomsurface by a normalized hydrogen concentration H/Si in the same surface,from a value obtained by dividing an ion exchange amount 1 in the topsurface by a normalized hydrogen concentration H/Si in the same surface,

where the ion exchange amount 1 is determined according to the followingformula:

Ion exchange amount 1=5.51×(normalized Na₂O surfaceconcentration)−0.038×(Sn concentration)

in the formula, the normalized Na₂O surface concentration is a valueobtained by dividing an Na₂O concentration at the surface by an Na₂Oconcentration at a depth position of 100 μm, where the Na₂Oconcentration is a value measured by a fluorescent X-ray analysis usingan Na-Kα ray, and the Sn concentration is an Sn deposition amount (unit:as SnO₂μg/cm²) per unit area of the top surface and the bottom surface;

and

where the normalized hydrogen concentration is a value obtained bydividing an average hydrogen concentration at a depth of 0 to 10 μm byan average hydrogen concentration at a depth of 105 to 110 μm, in whichthe average hydrogen concentration at a depth of 0 to 10 μm and theaverage hydrogen concentration at a depth of 105 to 110 μm are valuesmeasured under the following analysis conditions:

(Analysis Conditions)

Measurement apparatus: secondary ion mass spectrometry apparatus with aquadrupole mass spectrometer

Primary ion species: Cs⁺

Primary accelerating voltage: 5.0 kV

Primary ion current: 1 μA

Primary ion incident angle (angle from direction perpendicular to samplesurface): 60°

Luster size: 200×200 μm²

Detection region: 40×40 μm²

Secondary ion polarity: minus

Electron gun for neutralization: used.

5. The float glass for chemical strengthening according to any one ofthe above 1 to 4, containing a bottom surface coming into contact with amolten metal at the time of forming and a top surface opposing thebottom surface, in which W3 determined according to the followingformula (5-1) is 58 or less:

W3=744×[(ΔN—Na₂O)+0.01×(Sn concentration difference)]  formula (5-1)

in formula (5-1),

the ΔN—Na₂O is a value determined by subtracting a normalized Na₂Osurface concentration of the bottom surface which is a value obtained bydividing an Na₂O concentration at the surface in the bottom surface byan Na₂O concentration at a depth position of 100 μm therefrom, from thenormalized Na₂O surface concentration of the top surface which is avalue obtained by dividing an Na₂O concentration at the surface in thetop surface by an Na₂O concentration at a depth position of 100 μmtherefrom, where each Na₂O concentration is a value measured by afluorescent X-ray analysis using an Na-Kα ray;

andin formula (5-1),

the Sn concentration difference is a difference obtained by subtractingan Sn deposition amount (unit: as SnO₂μg/cm²) per unit area of the topsurface from an Sn deposition amount (unit: as SnO₂μg/cm²) per unit areaof the bottom surface, and in the case where the glass does not containSnO₂, this is equivalent to the Sn deposition amount per unit area ofthe bottom surface.

6. The float glass for chemical strengthening according to any one ofthe above 1 to 4, containing a bottom surface coming into contact with amolten metal at the time of forming and a top surface opposing thebottom surface, in which Δion exchange amount 2 which is a valueobtained by subtracting an ion exchange amount 2 in the bottom surfacefrom an ion exchange amount 2 in the top surface is 0.33 or less,

where the ion exchange amount 2 is a value determined according to thefollowing formula (6-1):

Ion exchange amount 2=−0.02×(H/Si)+5.54×(N—Na₂O concentration)−0.037×(Snconcentration)  formula (6-1)

in formula (6-1),

the H/Si is a normalized hydrogen concentration, where the normalizedhydrogen concentration is a value obtained by dividing an averagehydrogen concentration at a depth of 0 to 10 μm by an average hydrogenconcentration at a depth of 105 to 110 μm, in which the average hydrogenconcentration at a depth of 0 to 10 μm and the average hydrogenconcentration at a depth of 105 to 110 μm are values measured under thefollowing analysis conditions:

(Analysis Conditions)

Measurement apparatus: secondary ion mass spectrometry apparatus with aquadrupole mass spectrometer

Primary ion species: Cs⁺

Primary accelerating voltage: 5.0 kV

Primary ion current: 1 μA

Primary ion incident angle (angle from direction perpendicular to samplesurface): 60°

Luster size: 200×200 μm²

Detection region: 40×40 μm²

Secondary ion polarity: minus

Electron gun for neutralization: used;

andin formula (6-1),

the N—Na₂O concentration is a normalized Na₂O surface concentrationwhich is a value obtained by dividing a surface Na₂O concentration by anNa₂O concentration at a depth position of 100 μm, where the Na₂Oconcentration is a value measured by a fluorescent X-ray analysis usingan Na-Kα ray, and

the Sn concentration is an Sn deposition amount (unit: as SnO₂μg/cm²)per unit area.

7. The float glass for chemical strengthening according to the above 1,containing a bottom surface coming into contact with a molten metal atthe time of forming and a top surface opposing the bottom surface, inwhich a square (ΔN—Na₂O)² of a difference ΔN—Na₂O determined bysubtracting a normalized Na₂O surface concentration of the bottomsurface which is a value obtained by dividing an Na₂O concentration inthe bottom surface by an Na₂O concentration at a depth position of 100μm therefrom, from a normalized Na₂O surface concentration of the topsurface which is a value obtained by dividing an Na₂O concentration inthe top surface by an Na₂O concentration at a depth position of 100 μmtherefrom, is 5.0×10⁻⁴ or less, where each Na₂O concentration is a valuemeasured by a fluorescent X-ray analysis using an Na-Kα ray.8. The float glass for chemical strengthening according to the above 1,which is used for chemical strengthening in which a chemicalstrengthening temperature is T (unit: K) and a chemical strengtheningtime is t (unit: hours) and contains SiO₂, in which a dol determinedaccording to the following formula by using respective contents in mass% of SiO₂, Al₂O₃, MgO, CaO, SrO, BaO, ZrO₂, Na₂O and K₂O is 20 or less:

dol=−0.13×Al₂O₃−1.88×MgO−2.41×CaO−1.85×SrO−1.35×BaO−1.59×ZrO₂+1.50×Na₂O+2.42×K₂O−129359/T+9.28×t^(0.5)+182.88.

Al₂O₃, MgO, CaO, SrO, BaO, ZrO₂, Na₂O and K₂O are not essentialcomponents. A salt used for chemical strengthening typically containsKNO₃ in a concentration of from 95 to 100 mass %.

9. The float glass for chemical strengthening according to the above 1,which contains, in mass %, from 60 to 80% of SiO₂, from 0 to 8% ofAl₂O₃, from 8 to 22% of Na₂O, from 0 to 7% of K₂O, from 0 to 17% of MgO,from 0 to 22% of CaO, from 0 to 8% of SrO, from 0 to 8% of BaO, and from0 to 5% of ZrO₂. Here, for example, containing “from 0 to 7% of K₂O”means that K₂O is not essential but it may be contained up to 7%.

Preferable composition range is from 64 to 77% of SiO₂, from 0.01 to 7%of Al₂O₃, from 10 to 18% of Na₂O, from 0 to 5% of K₂O, from 1 to 10% ofMgO, from 1 to 12% of CaO, from 0 to 5% of SrO, from 0 to 5% of BaO, andfrom 0 to 3% of ZrO₂.

10. The float glass for chemical strengthening according to the above 1,containing, in mass %, from 60 to 80% of SiO₂, from 0.01 to 8% of Al₂O₃,from 8 to 22% of Na₂O, from 0 to 7% of K₂O and from 0 to 5% of ZrO₂, inwhich in the case of containing MgO, CaO, SrO or BaO, the total of theMgO, CaO, SrO and BaO contents is from 5 to 25%, and a ratio Na₂O/Al₂O₃of Na₂O and Al₂O₃ contents is 1.5 or more.11. The float glass for chemical strengthening according to the above10, in which the Na₂O/Al₂O₃ is 6 or less.12. The float glass for chemical strengthening according to the above 9,containing CaO, SrO or BaO, in which the total of the CaO, SrO and BaOcontents is from 1 to 7%.13. A method for producing a chemically strengthened glass having adepth of compressive stress layer of 20 μm or less, including chemicallystrengthening the glass for chemical strengthening described in theabove 1.

Advantage of the Invention

In the float glass for chemical strengthening of the present invention,the difference in the weathering degrees between the top surface and thebottom surface is small, so that even when the stress produced bychemical strengthening is not decreased or a polishing treatment, etc.before chemical strengthening is simplified or omitted, the warpage ofthe float glass after chemical strengthening can be reduced and anexcellent flatness can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the correlation between the normalized hydrogenconcentration [obtained by dividing the average H/Si at (H/Si) 0 to 10μm in SIMS analysis by the average H/Si at 105 to 110 μm] and thenormalized Na₂O surface concentration (obtained by dividing the surfaceNa₂O concentration in fluorescent X-ray analysis by the Na₂Oconcentration at a depth position of 100 μm) in the surface layer of asoda-lime glass sheet (raw sheet) before chemical strengthening.

FIG. 2 illustrates the mechanism in a glass before chemicalstrengthening, where Na⁺ in glass is ion-exchanged with H⁺ in theatmosphere.

FIG. 3 is a view showing the correlation between the ion exchange amount(K₂O, wt %) (fluorescent X-ray analysis) of a soda-lime glass sheetafter chemical strengthening and the normalized Na₂O surfaceconcentration (obtained by dividing the surface Na₂O concentration influorescent X-ray analysis by the Na₂O concentration at a depth positionof 100 μm) before chemical strengthening (raw sheet); where wt % is mass%.

FIG. 4 is a schematic view illustrating the method for calculating theion exchange amount in fluorescent X-ray analysis.

FIGS. 5A, 5B, 5C and 5D are schematic views illustrating the mechanismwhere when a soda-lime glass sheet in which Na⁺ and H⁺ are ion-exchangedis chemically strengthened by dipping it in a KNO₃ molten salt, the ionexchange amount decreases. FIG. 5A is a graph showing an amount of K₂Oin a raw sheet. FIG. 5B is a graph showing an amount of K₂O in achemical strengthened article. FIG. 5C is a graph showing the ionexchange amount. FIG. 5D illustrates how the ion exchange amount iscalculated.

FIG. 6 is a graph plotting, on the abscissa, the difference Δ(N—Na₂O²)(Top-Bottom) between squared normalized Na₂O surface concentrations intop surface and bottom surface of glass to be subjected to chemicalstrengthening, and on the ordinate, Δwarpage amount.

FIG. 7 is a graph plotting, on the abscissa, the difference between ionexchange amounts in top surface and bottom surface, and on the ordinate,Δwarpage amount.

FIG. 8 is a graph plotting, on the abscissa, the difference betweenvalues obtained by dividing the ion exchange amounts in top surface andbottom surface by the hydrogen concentration, and on the ordinate,Δwarpage amount.

FIG. 9 is a graph showing the results of multiple regression analysisusing, as factors, the normalized Na₂O surface concentration difference(ΔNa₂O) and Sn concentration difference (deposition amount per unitarea) in the top surface and bottom surface before chemicalstrengthening, and the Δwarpage amount.

FIG. 10 is a longitudinal cross-sectional view of the apparatus forproducing the float glass for chemical strengthening of the presentinvention.

FIG. 11 is a cross-sectional view of a flat panel display in which thefloat glass for chemical strengthening of the present invention ischemically strengthened and then used as a cover glass for a flat paneldisplay.

FIG. 12 shows a graph plotting W1 on the abscissa and Δwarpage amount onthe ordinate.

FIG. 13 shows a graph plotting W2 on the abscissa and Δwarpage amount onthe ordinate.

FIG. 14 shows a graph plotting W3 on the abscissa and Δwarpage amount onthe ordinate.

FIG. 15 shows a graph plotting, on the abscissa, the difference (Δionexchange amount 2) between ion exchange amounts 2 in top surface andbottom surface, and on the ordinate, Δwarpage amount.

FIG. 16 shows a graph plotting, on the abscissa, a square [ΔN—Na₂O(Top-Bottom)]² of the difference obtained by subtracting the normalizedNa₂O surface concentration of the bottom surface from the normalizedNa₂O surface concentration of the top surface, and on the ordinate,Δwarpage amount.

FIG. 17 shows a graph plotting [(ΔN—Na₂O)+0.01×(Sn concentrationdifference)] on the abscissa and W3 on the ordinate.

MODE FOR CARRYING OUT THE INVENTION 1. Weathering of Glass

The H profile (¹H⁻/³⁰Si⁻) on the surface of a soda-lime glass producedby a float method was analyzed by a secondary ion mass spectrometryapparatus (SIMS), and as a result, the depth of weathered (hydrated anddealkalized) layer was about 3 μm. This implies that at the time ofchemical strengthening to an ion-exchange depth of 20 μm or less, as acause of warpage occurring due to the difference in the chemicalstrengthening behaviors between the bottom surface and the top surface,the difference between weathering degrees in the top surface and thebottom surface is important.

The “weathering” as used in the present invention indicates a phenomenonwhere the glass surface is degraded due to corrosion of the glasssurface by the atmosphere, usually, by the effect of humidity, and inthe present invention, this indicates a phenomenon where an alkali metalcomponent, typically, Na₂O, in the surface layer of glass is desorbed.The weathering degree of glass can be analyzed by measuring the Na₂Oconcentration by fluorescent X-ray analysis.

FIG. 1 illustrates the correlation between the normalized hydrogenconcentration (SIMS analysis) and the normalized Na₂O surfaceconcentration (obtained by dividing the surface Na₂O concentration influorescent X-ray analysis by the Na₂O concentration at a depth positionof 100 μm) in the surface layer of a soda-lime glass sheet (raw sheet)before chemical strengthening. As shown in FIG. 1, in the surface layerof a soda-lime glass sheet before chemical strengthening, the normalizedhydrogen concentration is in an inverse relationship with the normalizedNa₂O surface concentration.

The graph of FIG. 1 indicates that as explained in FIG. 2, in the glassbefore chemical strengthening, Si—O—Na constituting the glass reactswith H₂O in the atmosphere and Na⁺ and H⁺ are ion-exchanged.Accordingly, the normalized hydrogen concentration in the glass surfacelayer is considered to increase as the weathering degree of glass islarger.

FIG. 3 illustrates the correlation between the ion exchange amount (wt%) (fluorescent X-ray analysis) of a soda-lime glass sheet afterchemical strengthening and the normalized Na₂O surface concentration ofa raw sheet. Here, with regard to the ion exchange amount, as shown inFIG. 4, the value obtained by subtracting the K₂O analysis value beforechemical strengthening (raw sheet) from the K₂O analysis value afterchemical strengthening is defined as the ion exchange amount.

It is seen from the graph of FIG. 3 that as the Na₂O concentration inglass before chemical strengthening is higher, namely, as the weatheringdegree of glass is smaller, the ion exchange amount after chemicalstrengthening increases.

The graph of FIG. 3 also implies the following. That is, as shown inFIGS. 5A, 5B, 5C and 5D, when a soda-lime glass sheet in which Na⁺ andH⁺ are ion-exchanged as shown in FIG. 2 is chemically strengthened bydipping it in a KNO₃ molten salt, the ion exchange of Na⁺ in glass withK⁺ is governed by entropy but in the case of exchange of H⁺ with K⁺, theion exchange is considered to be not driven enthalpically, because H inglass is present as SiOH (weak acid) and if H⁺ and K⁺ are ion-exchanged,HNO₃ (strong acid) is produced.

Accordingly, it is thought that the weathering degree of a soda-limeglass before chemical strengthening affects the ion exchange amount andthe difference in the ion exchange amount between the top surface andthe bottom surface causes the generation of warpage after chemicalstrengthening. This leads to an understanding that in order to suppresswarpage of a soda-lime glass after chemical strengthening, it isimportant to control the difference (the Na₂O concentration differencebetween the top surface and the bottom surface) in the weatheringdegrees of the glass surface layer between top surface and bottomsurface of glass before chemical strengthening.

2. Sn Concentration

The Sn (tin) profile (¹²⁰Sn⁻/³⁰Si⁻) in the bottom surface of a soda-limeglass produced by a float method was analyzed by a secondary ion massspectrometry apparatus (SIMS), and as a result, the depth ofion-exchanged layer and the Sn invasion depth were about 7 μm. Thisimplies that at the time of chemical strengthening of low DOL to anion-exchange depth, i.e., DOL, of 20 μm or less, as a cause of warpageoccurring due to the difference in the chemical strengthening behaviorsbetween the bottom surface and the top surface, the Sn concentrationneeds to be taken into account.

In this connection, both Δ(N—Na₂O²) and (ΔN—Na₂O)² depend on thedifference of weathering degrees but are not dependent directly on theSn concentration. However, invasion of Sn into the bottom surface in afloat bath is considered to occur due to ion-exchange with Na of theglass surface layer. Therefore, in glass having a large Sn depositionamount, the Na concentration of the surface layer is assumed to be low.Accordingly, the normalized Na₂O surface concentration has arelationship with the Sn concentration. In other words, although notexplicit, both Δ(N—Na₂O²) and (ΔN—Na₂O)² can be said to depend on the Snconcentration.

At the time of float forming, when glass is densified by the invasion ofSn into the glass, the pathway for ion-exchange of Na ion with K ion isnarrowed and, as a result, the ion exchange reaction is disturbed andthe chemical strengthening is inhibited in the Sn invaded surface(bottom surface). This is considered to cause difference in the chemicalstrengthening behaviors between the top surface and the bottom surface,leading to warpage of the glass.

The Sn concentration of glass can be determined by measuring the Sndeposition amount per unit area. Specifically, it can be determined, forexample, by etching the glass with a hydrofluoric acid solution andquantitatively determining the Sn concentration in the solution by ICPemission spectrometry.

3. Hydrogen Concentration

The float glass for chemical strengthening of the present invention isformed by a float method and has a bottom surface coming into contactwith a molten metal at the time of forming and a top surface opposingthe bottom surface. As described below, the hydrogen concentrationdifference between the top surface and the bottom surface is consideredto sometimes work out to one of causes of warpage that is caused bychemical strengthening of the float glass.

In the production of glass by a float method, a glass plate is producedby continuously feeding onto a surface of a molten metal retained in afloat bath, a molten glass from an upstream side to form a glass ribbon,concurrently drawing the formed glass ribbon from a downstream side endof the float bath, and annealing it in a lehr.

In the production of glass by a float method, an apparatus usually usedin a type where a glass tank furnace and a float bath are connectedthrough a canal and a spout and the flow passage is narrowed down. Inthis case, since the glass must be spread in the float bath, a moltenglass at a higher temperature than in the case of another type ofapparatus described later is poured onto the molten metal surface andformed.

At this time, when the hydrogen concentration in glass is high, hydrogenenters as the form of SiOH into the Si—O—Si bond network of glass, andthe Si—O—Si bond is broken. When the hydrogen concentration in glass ishigh, the Si—O—Si bond is broken in many portions and since this leadsto deterioration of thermal characteristics such as glass transitiontemperature, stress relaxation occurs at the time of chemicalstrengthening in which the glass is heated at a high temperature,resulting in decrease of the stress.

For this reason, the degree of stress produced at the time of chemicalstrengthening is low in a glass surface having a higher hydrogenconcentration, out of the top surface and bottom surface in a floatglass, and a stress is likely to be produced at the time of chemicalstrengthening in a glass surface having a lower hydrogen concentration.

In other words, it is considered that when a float glass having a lowerhydrogen concentration in the top surface than in the bottom surface ischemically strengthened, a higher stress is produced in the top surfacehaving a low hydrogen concentration than in the bottom surface having ahigh hydrogen concentration, and the glass is warped to project towardthe top surface side, resulting in occurrence of warpage.

Therefore, as the hydrogen concentration in the top surface of a floatglass is closer to that in the bottom surface, i.e., as the absolutevalue of the hydrogen concentration difference between the top surfaceand the bottom surface is smaller, the degree of stress produced comesclose to an equilibrium state between the top surface and the bottomsurface after chemical strengthening, and therefore the warpage isdecreased.

In the present invention, since the average hydrogen concentrationitself and the average hydrogen concentration difference itself canhardly be measured with good precision, the [¹H⁻/³⁰Si⁻] (also referredto as H/Si) proportional to the average hydrogen concentration is usedas a direct indicator of the average hydrogen concentration, and the“difference in the normalized hydrogen concentrations between the topsurface and the bottom surface” and the “difference in the normalizedintensities between the top surface and the bottom surface”, which areproportional to the average hydrogen concentration difference above, areused as a direct indicator of the average hydrogen concentrationdifference.

In the present specification, the [¹H⁻/³⁰Si⁻] is a value measured underthe following analysis conditions.

(Analysis Conditions)

Measurement apparatus: secondary ion mass spectrometry apparatus with aquadrupole mass spectrometer

Primary ion species: Cs⁺

Primary accelerating voltage: 5.0 kV

Primary ion current: 1 μA

Primary ion incident angle (angle from direction perpendicular to samplesurface): 60°

Luster size: 200×200 μm²

Detection region: 40×40 μm²

Secondary ion polarity: minus

Electron gun for neutralization: used

The [¹H⁻/³⁰Si⁻], normalized intensity and normalized hydrogenconcentration are described below. The secondary ion intensity I_(M1) ofisotope M₁ of element M in secondary ion mass spectrometry isproportional to the primary ion intensity I_(P), sputter ratio Y ofmatrix, concentration C_(M) (ratio to the total concentration) ofelement M, existence probability α₁ of isotope M₁, secondary ionizationratio β_(M) of element M, and transmission efficiency η (includingdetection efficiency of detector) of mass spectrometer.

I _(M1) =A·I _(P) ·Y·C _(M)·α₁·β_(M)·η  (formula 1)

In the formula, A is the ratio of detection area of secondary ion toscanning range of primary ion beam. In general, η of an apparatus can behardly determined and in turn, the absolute value of β_(M) cannot beobtained. Therefore, η is eliminated by using the main component elementor the like in the same sample as a reference element and employing theratio to (formula 1).

Assuming that the reference element is R and its isotope is R_(J),(formula 2) is established:

I _(M1) /I _(Rj)=(C _(M)·α₁·β_(M))/(C _(R)·α_(j)·β_(R))=C _(M)/K  (formula 2)

in which K is a relative sensitivity factor of element M to element R.

K=(C _(R)·α_(j)·β_(R))/(α₁·β_(M))  (formula 3)

In this case, the concentration of element M is determined according to(formula 4):

C _(M) =K·I _(M1) /I _(Rj)  (formula 4)

In the present invention, ¹H⁻ and ³⁰Si⁻ correspond to M₁ and R_(j),respectively. Therefore, from the (formula 2), the intensity ratio[¹H⁻/³⁰Si⁻] of those two is equivalent to a value obtained by dividingthe average hydrogen concentration C_(H) by K. That is, the [¹H⁻/³⁰Si⁻]is a direct indicator of the average hydrogen concentration.

The normalized intensity is a value obtained by dividing [¹H⁻/³⁰Si⁻] ata certain depth x by [¹H⁻/³⁰Si⁻] at a depth of 105 to 110 μm, that is, avalue obtained by dividing C_(H)/K at a certain depth x by C_(H)/K at adepth of 105 to 110 μm. Since K is eliminated, the normalized intensityis eventually the same as a value obtained by dividing C_(H) at a depthx by C_(H) at a depth of 105 to 110 μm and, namely, is the normalizedhydrogen concentration at a depth x.

The reason why the average hydrogen concentration at a depth of 105 to110 μm is employed as the basis when calculating the normalized hydrogenconcentration is because the region at a depth of 105 to 110 μm isconsidered to be an inner region where the average hydrogenconcentration does not vary.

The absolute value of the normalized intensity difference between thetop surface and the bottom surface in a float glass is obtained bysecondary ion mass spectrometry (SIMS analysis), for example, throughthe following procedures (i) to (iii). Here, the analysis conditionsdescribed below are for exemplification and should be appropriatelychanged according to the measurement apparatus, sample, etc.

(i) In each of the top surface and the bottom surface, the secondary ionmass spectrometry is performed to a depth of 20 μm from the surfacelayer under the following analysis conditions.

(Analysis Conditions)

Measurement apparatus: secondary ion mass spectrometry apparatus with aquadrupole mass spectrometer

Primary ion species: Cs⁺

Primary accelerating voltage: 5.0 kV

Primary ion current: 1 μA

Primary ion incident angle (angle from direction perpendicular to samplesurface): 60°

Luster size: 200×200 μm²

Detection region: 40×40 μm²

Secondary ion polarity: minus

Electron gun for neutralization: used

In the case where the intensity of ³⁰Si⁻ at a depth of 55 μm is smallerthan the intensity of ³⁰Si⁻ at a depth of 5 μm by more than 3%, theanalysis is preferably performed on a sample in which the surface of aglass substrate is previously etched by about 45 μm.

More specific analysis conditions are, for example, as follows.

(Analysis Conditions)

Measurement apparatus: secondary ion mass spectrometry apparatus with aquadrupole mass spectrometer

Primary ion species: Cs⁺

Primary accelerating voltage: 5.0 kV

Primary ion current: 1 μA

Primary ion incident angle (angle from direction perpendicular to samplesurface): 60°

Luster size: 200×200 μm²

Detection region: 40×40 μm²

Sputter rate: 14 nm/sec

Secondary ion polarity: minus

Electron gun for neutralization: used

The secondary ion mass spectrometry apparatus with a quadrupole massspectrometer includes, for example, ADEPT 1010, manufactured byUlvac-Phi, Inc.

(ii) A value obtained by dividing [¹H⁻/³⁰Si⁻] at a depth of 0 to 10 μmin the [¹H⁻/³⁰Si⁻] profile obtained by secondary ion mass spectrometryby [¹H⁻/³⁰Si⁻] at a depth of 105 to 110 μm is defined as the normalizedintensity at a depth of 0 to 10 μm in secondary ion mass spectrometry.

(iii) With regard to the normalized intensity at a depth of 0 to 10 μmobtained by secondary ion mass spectrometry, the absolute value of thedifference between the top surface and the bottom surface is calculated.

4. Ion Exchange Amount

The ion exchange amount is a stress generation factor and is in aproportional relationship with the K₂O concentration in glass afterchemical strengthening. Therefore, the difference between ion exchangeamounts in the top surface and the bottom surface can be analyzed by theK₂O concentration difference. The K₂O concentration can be analyzed byfluorescent X-ray analysis.

5. Warpage Amount

The float glass for chemical strengthening of the present invention is afloat glass having a small amount of warpage after chemicalstrengthening. The warpage amount of the float glass can be measured bya contact-type surface profile analyzer [for example, SURFCOM (tradename), manufactured by Tokyo Seimitsu Co., Ltd.].

The warpage amount is measured as the difference between the highestpoint and the lowest point when measured with a contact-type surfaceprofile analyzer, after correcting the base line so that the measurementstart point and the measurement end point can be aligned at the samelevel. The warpage amount is indicated by a positive value when thefloat glass is warped in the convex direction to the top surface, and bya negative value when warped in the convex direction to the bottomsurface.

The change in the warpage amount of the float glass before and afterchemical strengthening can be measured according to the followingformula.

ΔWarpage amount=(warpage amount after chemical strengthening)−(warpageamount before chemical strengthening)  (formula)

In the present invention, the measurement is performed on a central 9cm-square region of a 10 cm-square float glass, and the absolute valueof Δwarpage amount in terms of sheet thickness of 07 mm is preferably 58μm or less, 56 μm or less, 54 μm or less, or 52 μm or less. When theabsolute value of Δwarpage amount is not more than the upper limitabove, the warpage after chemical strengthening can be decreased.

The CS (surface compressive stress) and DOL (depth of compressive stresslayer) can be measured by a surface stress meter. In regard to the floatglass for chemical strengthening of the present invention, the surfacecompressive stress of the chemically strengthened glass is preferably650 MPa or more, and it is preferably used when the depth of compressivestress layer is 20 μm or less. When the depth of compressive stresslayer is 20 μm or less, the product after chemical strengthening can becut, and this is preferred. In this viewpoint, the depth of compressivestress layer is more preferably 15 μm or less.

6. Parameters

The discussion above implies the following parameters.

(1) Na₂O Concentration Difference Between Top Surface and Bottom SurfaceBefore Chemical Strengthening, and ΔWarpage Amount

It is considered that for controlling the warpage after chemicalstrengthening of a soda-lime glass, it is important to control theweathering degree, hydrogen concentration and Sn concentration in theglass surface layer before chemical strengthening.

Here, as shown in FIG. 1, in the surface layer in glass before chemicalstrengthening, the normalized hydrogen concentration is in an inverserelationship with the normalized Na₂O surface concentration. Inaddition, as shown in FIG. 3, as the normalized Na₂O surfaceconcentration in glass before chemical strengthening is higher, the ionexchange amount after chemical strengthening is increased, and thus, thenormalized Na₂O surface concentration in glass before chemicalstrengthening is in a proportional relationship with the ion exchangeamount.

Furthermore, when the increase in the hydrogen concentration of theglass surface layer is large, the Si—O—Si bond is broken in manyportions and since this leads to deterioration of thermalcharacteristics such as glass transition temperature, stress relaxationoccurs at the time of chemical strengthening in which the glass isheated at a high temperature, resulting in decrease of the stress.Therefore, it can be considered that stress generation resulting fromchemical strengthening is attributable to the ion exchange amount andthe relaxation degree. For this reason, the normalized Na₂O surfaceconcentration difference between top surface and bottom surface in glassbefore chemical strengthening is considered to have a correlation withthe Δwarpage amount.

(1A) Difference Between Squared Normalized Na₂O Surface Concentrationsin Top Surface and Bottom Surface, and ΔWarpage Amount

FIG. 6 shows a graph plotting, on the abscissa, the differenceΔ(N—Na₂O²) (Top-Bottom) obtained by subtracting the squared normalizedNa₂O surface concentration of the bottom surface from the squarednormalized Na₂O surface concentration of the top surface, and on theordinate, Δwarpage amount.

It is seen from the graph shown in FIG. 6 that the difference Δ(N—Na₂O²)between squared normalized Na₂O surface concentrations in the topsurface and the bottom surface of a soda-lime glass sheet beforechemical strengthening and the Δwarpage amount have a correlationrepresented by the following formula (1-1).

ΔWarpage amount=370×Δ(N—Na₂O²)+45  formula (1-1)

In formula (1-1), the Δ(N—Na₂O²) is the difference between squares ofthe values obtained by measuring, by fluorescent X-ray analysis, thenormalized Na₂O surface concentrations in the top surface and the bottomsurface of glass to be subjected to chemical strengthening and isdetermined according to the following formula (1-2).

Δ(N—Na₂O²)=(normalized Na₂O surface concentration in top surface beforechemical strengthening)²−(normalized Na₂O surface concentration inbottom surface before chemical strengthening)²  formula (1-2)

Here, the normalized Na₂O surface concentration is a value obtained bydividing the surface Na₂O concentration by the Na₂O concentration at adepth position of 100 μm. Each Na₂O concentration is a value calculatedfrom the relative intensity ratio to a standard sample by measuring theintensity of Na-Kα ray by fluorescent X-ray analysis. Incidentally, theNa₂O concentration at a depth position of 100 μm is an Na₂Oconcentration obtained by measuring the surface with a fluorescent X-rayafter grinding off the glass to a depth of 100 μm from the surface. Inaddition, the analysis depth of the value measured by fluorescent X-rayanalysis using an Na-Kα ray is typically 3 μm.

The difference between squared normalized Na₂O surface concentrations inthe top surface and the bottom surface of glass to be subjected tochemical strengthening is 0.040 or less, preferably 0.035 or less, 0.030or less, or 0.025 or less. When the difference between squarednormalized Na₂O surface concentrations in the top surface and the bottomsurface of glass to be subjected to chemical strengthening is 0.040 orless, even if a polishing treatment, etc. before chemical strengtheningis simplified or omitted, warpage of the float glass after chemicalstrengthening can be reduced and excellent flatness can be obtained.

(1B) Square of Difference Between Normalized Na₂O Surface Concentrationsin Top Surface and Bottom Surface, and ΔWarpage Amount

FIG. 16 shows a graph plotting, on the abscissa, a square [ΔN—Na₂O(Top-Bottom)]² of the difference determined by subtracting thenormalized Na₂O surface concentration of the bottom surface which is avalue obtained by dividing the Na₂O concentration in bottom surface bythe Na₂O concentration at a depth position of 100 μm, from thenormalized Na₂O surface concentration of the top surface which is avalue obtained by dividing the Na₂O concentration in the top surface bythe Na₂O concentration at a depth position of 100 μm, and on theordinate, Δwarpage amount.

It is seen from the graph shown in FIG. 16 that the square [ΔN—Na₂O(Top-Bottom)]² of the difference determined by subtracting thenormalized Na₂O surface concentration of the bottom surface from thenormalized Na₂O surface concentration of the top surface and theΔwarpage amount have a correlation represented by the following formula(7-1).

ΔWarpage amount=18000×(ΔN—Na₂O)²+51  formula (7-1)

In formula (7-1), the (ΔN—Na₂O)² is a square of the difference betweenthe values obtained by measuring, by fluorescent X-ray analysis, thenormalized Na₂O surface concentrations in the top surface and the bottomsurface of glass to be subjected to chemical strengthening and isdetermined according to the following formula (7-2).

(ΔN—Na₂O)²=[(normalized Na₂O surface concentration in top surface beforechemical strengthening)−(normalized Na₂O surface concentration in bottomsurface before chemical strengthening)]²  formula (7-2)

The square of the difference between normalized Na₂O surfaceconcentrations in the top surface and the bottom surface of glass to besubjected to chemical strengthening is 5.0×10⁻⁴ or less, preferably4.5×10⁻⁴ or less, 4.0×10⁻⁴ or less, or 3.5×10⁻⁴ or less. When the squareof the difference between normalized Na₂O surface concentrations in thetop surface and the bottom surface of glass to be subjected to chemicalstrengthening is 5.0×10⁻⁴ or less, even if a polishing treatment, etc.before chemical strengthening is simplified or omitted, warpage of thefloat glass after chemical strengthening can be reduced and excellentflatness can be obtained.

The Na₂O concentration is adjusted by the method described later in (A)of “7. Production Method of Glass”, whereby the normalized Na₂O surfaceconcentrations in the top surface and the bottom surface of glass to besubjected to chemical strengthening can be adjusted, and Δ(N—Na₂O²) or(ΔN—Na₂O)² can be adjusted. Specifically, it is preferable, for example,to decrease the Na₂O concentration in the top surface by spraying watervapor or SO₂ gas onto the top surface at the time of annealing the glassor increase the Na₂O concentration in the bottom surface by lowering theflow rate of SO₂ gas sprayed onto the bottom surface for the purpose ofscratch prevention.

(2) Ion Exchange Amount Difference Between Top Surface and BottomSurface after Chemical Strengthening and ΔWarpage Amount

The Δion exchange amount 1 that is the difference between ion exchangeamounts in the top surface and the bottom surface after chemicalstrengthening is considered to have a correlation with the Δwarpageamount.

The ion exchange amount is proportional to the Na₂O concentration beforechemical strengthening and inhibited by Sn and therefore, can bedetermined according to the following formula (2-1).

Ion exchange amount 1=5.51×(normalized Na₂O surfaceconcentration)−0.038×(Sn concentration)  formula (2-1)

Hereinafter, the term “ion exchange amount” is sometimes used forindicating the ion exchange amount 1.

In formula (2-1), the normalized Na₂O surface concentration is a valueobtained by dividing the surface Na₂O concentration by the Na₂Oconcentration at a depth position of 100 μm. Here, each Na₂Oconcentration is a value measured by fluorescent X-ray analysis using anNa-Kα ray. In addition, the Sn concentration is an Sn deposition amount(unit: as SnO₂μg/cm²) per unit area of the top surface and the bottomsurface.

The difference between ion exchange amounts 1 in the top surface and thebottom surface can be determined according to the following formula(2-2).

Ion exchange amount 1 difference=(ion exchange amount 1 in topsurface)−(ion exchange amount 1 in bottom surface)  formula (2-2)

FIG. 7 shows a graph plotting, on the abscissa, Δion exchange amount 1that is the difference between ion exchange amounts in the top surfaceand the bottom surface, and on the ordinate, Δwarpage amount. It is seenfrom the graph shown in FIG. 7, the Δion exchange amount 1 and theΔwarpage amount have a correlation represented by the following formula(2-3).

ΔWarpage amount=103×(Δion exchange amount 1)+24  formula (2-3)

The Δion exchange amount 1 is 0.32 or less, preferably 0.30 or less,0.28 or less, 0.26 or less, or 0.24 or less.

When the difference between ion exchange amounts 1 in the top surfaceand the bottom surface after chemical strengthening, determinedaccording to formulae (2-1) and (2-2), is 0.32 or less, even if apolishing treatment, etc. before chemical strengthening is simplified oromitted, warpage of the float glass after chemical strengthening can bereduced and excellent flatness can be obtained.

The Na₂O concentration and the Sn concentration in the bottom surfaceare adjusted by the method described later in (A) of “7. ProductionMethod of Glass”, whereby the Δion exchange amount 1 that is thedifference between ion exchange amounts 1 in the top surface and thebottom surface after chemical strengthening can be adjusted.Specifically, it is preferable, for example, to decrease the Na₂Oconcentration in the top surface by spraying water vapor or SO₂ gas ontothe top surface at the time of annealing the glass, increase the Na₂Oconcentration in the bottom surface by lowering the flow rate of SO₂ gassprayed onto the bottom surface for the purpose of scratch prevention,or decrease the invasion amount of Sn into the bottom surface bylowering the temperature upstream of the float bath or increasing thehydrogen concentration in the atmosphere.

(3) Correlation of Hydrogen Concentration Difference and SnConcentration Difference Between Top Surface and Bottom Surface BeforeChemical Strengthening and Ion Exchange Amount Difference with ΔWarpageAmount

When multiple regression analysis is performed using, as factors, thehydrogen concentration difference, Sn concentration difference and ionexchange amount difference between the top surface and the bottomsurface before chemical strengthening, and the Δwarpage amount, thefollowing formula (3-1) is obtained.

W1=−16×(ΔH/Si)−6.47×(Sn concentration difference)−43.8×(Δion exchangeamount 1)  formula (3-1)

In formula (3-1), the ΔH/Si is the difference (difference betweennormalized hydrogen concentrations) between values obtained bymeasuring, by SIMS analysis, the hydrogen concentration differencebetween the top surface and the bottom surface before chemicalstrengthening and is determined according to the following formula(3-2).

ΔH/Si=(normalized hydrogen concentration in top surface before chemicalstrengthening)−(normalized hydrogen concentration in bottom surfacebefore chemical strengthening)  formula (3-2)

In formula (3-1), the Sn concentration difference is the differenceobtained by subtracting the Sn deposition amount (unit: as SnO₂μg/cm²)per unit area of the top surface from the Sn deposition amount (unit: asSnO₂μg/cm²) per unit area of the bottom surface, and in the case wherethe glass does not contain SnO₂, this is equivalent to the Sn depositionamount per unit area of the bottom surface.

The Δion exchange amount 1 is a value obtained by subtracting the ionexchange amount in the bottom surface from the ion exchange amount 1 inthe top surface. The ion exchange amount is determined according to theabove-mentioned formula (2-1).

FIG. 12 shows a graph plotting W1 on the abscissa and Δwarpage amount onthe ordinate. It is seen from the graph shown in FIG. 12 that W1 has acorrelation with the Δwarpage amount.

In formula (3-1), W1 is 56 or less, preferably 54 or less, 52 or less,or 50 or less. When W1 is 56 or less, even if a polishing treatment,etc. before chemical strengthening is simplified or omitted, warpage ofthe float glass after chemical strengthening can be reduced andexcellent flatness can be obtained.

The hydrogen concentration and the Sn concentration in the bottomsurface are adjusted by the method described later in (A) of “7.Production Method of Glass”, whereby W1 in formula (3-1) can beadjusted. Specifically, it is preferable, for example, to decrease theNa₂O concentration in the top surface by spraying water vapor or SO₂ gasonto the top surface at the time of annealing the glass, increase theNa₂O concentration in the bottom surface by lowering the flow rate ofSO₂ gas sprayed onto the bottom surface for the purpose of scratchprevention, or decrease the invasion amount of Sn into the bottomsurface by lowering the temperature upstream of the float bath orincreasing the hydrogen concentration in the atmosphere.

(4) Correlation of Ion Exchange Amount Difference Between Top Surfaceand Bottom Surface and Hydrogen Concentration Difference Between TopSurface and Bottom Surface Before Chemical Strengthening with ΔWarpageAmount

The ion exchange amount is thought to be a stress generation factor, andthe hydrogen concentration in the glass surface layer is thought to be astress relaxation factor.

Namely, it is considered that as the hydrogen concentration in the glasssurface layer is increased, the density of glass decreases. Since H inglass is present in the state of SiOH and the SiOH is produced resultingfrom breakage of a continuous crosslinked structure Si—O—Si in theglass, an increase in the hydrogen concentration in the glass surfacelayer is considered to cause a decrease in the density of glass, leadingto stress relaxation.

The warpage of glass after chemical strengthening is thought to beattributable to the unbalance of stress difference between the topsurface and the bottom surface and therefore, the value obtained bydividing the ion exchange amount by the hydrogen concentration isthought to have a correlation with the warpage amount.

FIG. 8 shows a graph plotting, on the abscissa, the difference betweenvalues obtained by dividing the ion exchange amounts in the top surfaceand the bottom surface by the normalized hydrogen concentrations in thetop surface and the bottom surface before chemical strengthening,respectively, and on the ordinate, Δwarpage amount.

It is seen from the graph shown in FIG. 8 that the difference in thevalues obtained by dividing the ion exchange amounts (ion exchangeamount) in the top surface and the bottom surface by the normalizedhydrogen concentrations (H/Si) in the top surface and the bottom surfacebefore chemical strengthening, respectively, and the Δwarpage amounthave a correlation represented by the following formula (4-1).

In addition, when multiple regression analysis is performed using, asfactors, the normalized hydrogen concentration, ion exchange amountdifference and Δwarpage amount, the following formula (4-1) is obtained.

W2=9.18×Δ[(ion exchange amount)/(H/Si)]+49  formula (4-1)

In formula (4-1), the Δ[(ion exchange amount)/(H/Si)] is a valuedetermined by subtracting a value that is obtained by dividing the ionexchange amount in the bottom surface by the normalized hydrogenconcentration H/Si thereof, from a value that is obtained by dividingthe ion exchange amount in the top surface by the normalized hydrogenconcentration H/Si thereof.

In formula (4-1), the ion exchange amount is obtained according to theabove-mentioned formula (2-1).

FIG. 13 shows a graph plotting W2 on the abscissa and Δwarpage amount onthe ordinate. It is seen from the graph shown in FIG. 13 that W2 has acorrelation with the Δwarpage amount.

In formula (4-1), the absolute value of W2 is 56 or less, preferably 54or less, 52 or less, or 50 or less. When W2 is 56 or less, even if apolishing treatment, etc. before chemical strengthening is simplified oromitted, warpage of the float glass after chemical strengthening can bereduced and excellent flatness can be obtained.

The hydrogen concentration is adjusted by the method described later in(A) of “7. Production Method of Glass”, whereby W2 in formula (4-1) canbe adjusted. Specifically, it is preferable, for example, to decreasethe Na₂O concentration in the top surface by spraying water vapor or SO₂gas onto the top surface at the time of annealing the glass, increasethe Na₂O concentration in the bottom surface by lowering the flow rateof SO₂ gas sprayed onto the bottom surface for the purpose of scratchprevention, or decrease the invasion amount of Sn into the bottomsurface by lowering the temperature upstream of the float bath orincreasing the hydrogen concentration in the atmosphere.

(5) Na₂O Concentration Difference and Sn Concentration DifferenceBetween Top Surface and Bottom Surface Before Chemical Strengthening andΔWarpage Amount

In order to control warpage of a soda-lime glass after chemicalstrengthening, it is important to control the weathering degree of theglass surface layer and Sn concentration before chemical strengthening,and therefore, the Na₂O concentration difference and Sn concentrationdifference in the top surface and the bottom surface before chemicalstrengthening are considered to have a correlation with the Δwarpageamount.

When multiple regression analysis is performed using, as factors, thenormalized Na₂O surface concentration difference (ΔN—Na₂O) and Snconcentration difference between the top surface and the bottom surfacebefore chemical strengthening and the Δwarpage amount, as shown in FIG.9, the normalized Na₂O surface concentration difference and Snconcentration difference between the top surface and the bottom surfacebefore chemical strengthening have a correlation represented by thefollowing formula (5-1).

W3=744×[(ΔN—Na₂O)+0.01×(Sn concentration difference)]  formula (5-1)

In formula (5-1), the ΔN—Na₂O is the difference in the normalized Na₂Osurface concentrations that are values obtained by dividing the Na₂Oconcentrations at the surface in the top surface and the bottom surfaceof glass to be subjected to chemical strengthening by the Na₂Oconcentrations at a depth position of 100 respectively, and isdetermined according to the following formula (5-2). Here, each Na₂Oconcentration is a value measured by fluorescent X-ray analysis using anNa-Kα ray.

ΔN—Na₂O=(normalized Na₂O surface concentration in topsurface)−(normalized Na₂O surface concentration in bottomsurface)  formula (5-2)

In addition, the Sn concentration difference is an Sn concentrationdifference between the top surface and the bottom surface beforechemical strengthening and is the difference obtained by subtracting theSn deposition amount (unit: as SnO₂μg/cm²) per unit area of the topsurface from the Sn deposition amount (unit: as SnO₂μg/cm²) per unitarea of the bottom surface, and in the case where the glass does notcontain SnO₂, this is equivalent to the Sn deposition amount per unitarea of the bottom surface.

FIG. 14 shows a graph plotting W3 on the abscissa and Δwarpage amount onthe ordinate. It is seen from the graph shown in FIG. 14 that W3 has acorrelation with the Δwarpage amount.

In formula (5-1), W3 is 58 or less, preferably 56 or less, 54 or less,or 52 or less. When W3 is 58 or less, even if a polishing treatment,etc. before chemical strengthening is simplified or omitted, warpage ofthe float glass after chemical strengthening can be reduced andexcellent flatness can be obtained.

The Na₂O concentration difference and the Sn concentration differenceare adjusted by the method described later in (A) of “7. ProductionMethod of Glass”, whereby W3 in formula (5-1) can be adjusted.Specifically, it is preferable, for example, to decrease the Na₂Oconcentration in the top surface by spraying water vapor or SO₂ gas ontothe top surface at the time of annealing the glass, increase the Na₂Oconcentration in the bottom surface by lowering the flow rate of SO₂ gassprayed onto the bottom surface for the purpose of scratch prevention,or decrease the invasion amount of Sn into the bottom surface bylowering the temperature upstream of the float bath or increasing thehydrogen concentration in the atmosphere.

(6) Ion Exchange Amount (Hydrogen Concentration, Na₂O Concentration andSn Concentration) Difference and ΔWarpage Amount

The ion exchange amount difference between the top surface and thebottom surface affects the warpage of glass after chemicalstrengthening, and the hydrogen concentration, Na₂O concentration and Snconcentration are considered to be responsible for the ion exchangeamount. Accordingly, with the normalized hydrogen concentration,normalized Na₂O surface concentration and Sn concentration, the ionexchange amount shows a correlation represented by the following formula(6-1)

Ion exchange amount 2=−0.02×(H/Si)+5.54×(N—Na₂O concentration)−0.037×(Snconcentration)  formula (6-1)

FIG. 15 shows a graph plotting, on the abscissa, the difference (Δionexchange amount 2) between ion exchange amounts 2 in top surface andbottom surface, and on the ordinate, Δwarpage amount. It is seen fromthe graph shown in FIG. 15 that the Δion exchange amount 2 and theΔwarpage amount have a correlation.

The Δion exchange amount 2 is a value determined according to thefollowing formula (6-2)

ΔIon exchange amount 2=(Δion exchange amount 2 in top surface)−(Δionexchange amount 2 in bottom surface)  formula (6-2)

In formula (6-1), the Δion exchange amount 2 is 0.33 or less, preferably0.31 or less, 0.29 or less, or 0.27 or less. When the Δion exchangeamount 2 is 0.33 or less, even if a polishing treatment, etc. beforechemical strengthening is simplified or omitted, warpage of the floatglass after chemical strengthening can be reduced and excellent flatnesscan be obtained.

The hydrogen concentration, Na₂O concentration and Sn concentration areadjusted by the method described later in (A) of “7. Production Methodof Glass”, whereby the ion exchange amount 2 in formula (6-1) can beadjusted. Specifically, it is preferable, for example, to decrease theNa₂O concentration in the top surface by spraying water vapor or SO₂ gasonto the top surface at the time of annealing the glass, increase theNa₂O concentration in the bottom surface by lowering the flow rate ofSO₂ gas sprayed onto the bottom surface for the purpose of scratchprevention, or decrease the invasion amount of Sn into the bottomsurface by lowering the temperature upstream of the float bath orincreasing the hydrogen concentration in the atmosphere.

7. Production Method of Glass

The method for reducing the Δwarpage amount by achieving a smalldifference in the weathering degrees between the top surface and thebottom surface of a float glass and a small difference in the amounts ofmetal invading the glass surface put into contact with a molten metal atthe time of float forming includes, for example, methods described inthe following (A) to (D). These methods may be used individually or incombination.

(A) At the time of annealing a glass coming out of a float bath, byspraying SO₂ gas onto the glass, the alkali component Na₂O of the glassis taken out from the glass as Na₂SO₄. By adjusting the amount of SO₂gas sprayed onto the glass, the amounts of alkali in the top surface andthe bottom surface can be adjusted to the same level, and the differencein the weathering degree of glass can be reduced.

(B) Water vapor is sprayed on the top surface side of glass in a lehr.

(C) The residence time of molten glass in a float bath is shortened.

(D) The temperature in the upstream region of a float bath is lowered.

The present invention is described below based on the drawings, but thepresent invention is not limited thereto. FIG. 10 is a longitudinalcross-sectional view of a production apparatus for a float glass of thepresent invention. In FIG. 10, 12 is a tweel, 22 is a fixed refractorylocated below the tweel, and 23 is a spout lip.

Although omitted in the drawing, a raw material is continuously fed to aglass tank furnace to melt the raw material in a high temperature regioninside the glass tank furnace, and the molten glass obtained is guidedto a cooling region to control the temperature. The molten glass 1 atthe controlled temperature passes through a connection groove 11, passesthrough a space 2 formed by the tweel 12 and the fixed refractory 22located therebelow, then fed to a molten metal bath 5 through the spoutlip 23, and formed into a glass ribbon 4.

The sheet thickness of the float glass is preferably 1.5 mm or less,more preferably 1.1 mm or less. In addition, although it is typically0.7 mm or more, a glass thinner than that is also used, if desired.

The float glass for chemical strengthening of the present invention canreduce the warpage after chemical strengthening irrespective of thecomposition. The composition of the float glass for chemicalstrengthening includes, for example, the following glass compositions.

(i) A glass having a composition containing, in mass %, from 60 to 80%of SiO₂, from 0.01 to 8% of Al₂O₃, from 8 to 22% of Na₂O, from 0 to 7%of K₂O, from 5 to 25% in total of RO (R=Mg, Ca, Sr, Ba), and from 0 to5% of ZrO₂.

(ii) A glass having a composition containing, in mass %, from 64 to 77%of SiO₂, from 0.01 to 7% of Al₂O₃, from 10 to 18% of Na₂O, from 0 to 5%of K₂O, from 1 to 10% of MgO, from 1 to 12% of CaO, from 0 to 5% of SrO,from 0 to 5% of BaO, and from 0 to 3% of ZrO₂.

(iii) A glass having a composition containing, in mass %, from 60 to 80%of SiO₂, from 0.01 to 8% of Al₂O₃, from 8 to 22% of Na₂O, from 0 to 7%of K₂O, and from 0 to 5% of ZrO₂, where in the case of containing MgO,CaO, SrO or BaO, the total of MgO, CaO, SrO and BaO contents is from 5to 25% and the ratio Na₂O/Al₂O₃ of Na₂O and Al₂O₃ contents is 1.5 ormore.

(iv) A glass having a composition containing, in mass %, from 60 to 80%of SiO₂, from 0.01 to 8% of Al₂O₃, from 8 to 22% of Na₂O, from 0 to 7%of K₂O, and from 0 to 5% of ZrO₂, where in the case of containing MgO,CaO, SrO or BaO, the total of MgO, CaO, SrO and BaO contents is from 5to 25% and the ratio Na₂O/Al₂O₃ of Na₂O and Al₂O₃ contents is from 1.5to 6.

(v) A glass having a composition containing, in mass %, from 60 to 80%of SiO₂, from 0.01 to 8% of Al₂O₃, from 8 to 22% of Na₂O, from 0 to 7%of K₂O, and from 0 to 5% of ZrO₂, where in the case of containing MgO,CaO, SrO or BaO, the total of MgO, CaO, SrO and BaO contents is from 5to 25%, the total of CaO, SrO and BaO contents is from 1 to 7%, and theratio Na₂O/Al₂O₃ of Na₂O and Al₂O₃ contents is 1.5 or more.

The float glass formed is cut into a predetermined size by a cutter notshown and then chemically strengthened, whereby a chemicallystrengthened float glass can be obtained.

The chemical strengthening is a treatment of forming a compressivestress layer on a glass surface by ion exchange at a temperature notmore than a glass transition temperature, where an alkali metal ionhaving a small ion radius (typically, Li ion or Na ion) in a glasssurface is exchanged with an alkali ion having a larger ion radius(typically, K ion). The chemical strengthening treatment can beperformed by a conventionally known method.

An example where the float glass of the present invention is chemicallystrengthened and then used as a cover glass for a flat panel display isdescribed below. FIG. 11 is a cross-sectional view of a display devicewhere a cover glass is disposed. In the following description,front/back and right/left are based on the direction of the arrow in thedrawings.

As illustrated in FIG. 11, a display device 10 generally includes adisplay panel 20 provided in a housing 15, and a cover glass 30 providedto cover the entire surface of the display panel 20 and surround thefront of the housing 15.

The cover glass 30 is disposed mainly for the purpose of improving thebeauty and strength of the display device 10, preventing the impactdamage and the like and is formed from one sheet-shaped glass having awhole shape of nearly planer shape. As illustrated in FIG. 11, the coverglass 30 may be disposed to be spaced (to have an air layer) from thedisplay side (front side) of the display panel 20 or may be attached tothe display side of the display panel 20 with an adhesive film (notshown) having translucency.

On the front surface of the cover glass 30, which emits light from thedisplay panel 20, a functional film 41 is provided, and on the backsurface where light from the display panel 20 enters, a functional film42 is provided at a position corresponding to the display panel 20. InFIG. 11, the functional films 41 and 42 are provided on both surfaces,but not limited thereto, and they may be provided on the front surfaceor back surface or may be omitted.

The functional films 41 and 42 have a function, for example, ofpreventing reflection of surrounding light, preventing impact damage,shielding electromagnetic wave, blocking near infrared ray, correctingcolor tone, and/or enhancing scratch resistance, and the thickness,shape, etc. are appropriately selected according to usage. Thefunctional films 41 and 42 are formed, for example, by attaching aresin-made film to the cover glass 30. Alternatively, they may be formedby a thin film formation method such as deposition method, sputteringmethod or CVD method.

The reference numeral 44 is a black layer and is, for example, a coatingfilm formed by coating an ink containing a pigment particle on the coverglass 30 and subjecting it to ultraviolet irradiation or heating/firingand then cooling. The display panel, etc. is made invisible from theoutside of the housing 15 and thereby the aesthetics of appearance isenhanced. The reference numeral 44 is not limited to a black layer andmay be, for example, a white layer.

PRACTICAL EXAMPLES

Practical Examples of the present invention are specifically describedbelow, but the present invention is not limited thereto.

Practical Example 1 Production of Float Glass

Glass having the following composition was produced by a float method tohave a sheet thickness of 0.7 mm and cut into a size of 10 cm×10 cm toproduce float glass plates of Examples 1 to 4.

Composition A:

in mass %, SiO₂: 71.5%, Al₂O₃: 1.8%, Na₂O: 13.5%, K₂O: 0.26%, MgO:4.64%, CaO: 7.83%, ZrO₂: 0.03%

Composition B:

SiO₂: 71.5%, Al₂O₃: 1.8%, Na₂O: 13.5%, K₂O: 0.26%, MgO: 4.64%, CaO:7.83%, ZrO₂: 0.03%

Composition C:

SiO₂: 71.5%, Al₂O₃: 1.8%, Na₂O: 13.5%, K₂O: 0.26%, MgO: 4.64%, CaO:7.83%, ZrO₂: 0.03%

[Evaluation Methods] (1) Measurement of Hydrogen Concentration in GlassSurface Layer

The hydrogen concentration of each float glass of Practical Examples 1and 2 and Comparative Examples 1 to 3 was analyzed down to a depth of 20μm by secondary ion mass spectrometry. The [¹H⁻/³⁰Si⁻] profile of thefloat glass by secondary ion mass spectrometry is shown, and thisprofile can be regarded as equivalent to the hydrogen concentrationprofile.

Analysis conditions of the secondary ion mass spectrometry were asfollows.

Measurement apparatus: ADEPT 1010, manufactured by Ulvac-Phi, Inc.

Primary ion species: Cs⁺

Primary accelerating voltage: 5.0 kV

Primary ion current: 1 μA

Primary ion incident angle (angle from direction perpendicular to samplesurface): 60°

Luster size: 200×200 μm²

Detection region: 40×40 μm²

Sputter rate: 14 nm/sec

Secondary ion polarity: minus

Electron gun for neutralization: used

The [¹H⁻/³⁰Si⁻]'s at a depth of 0 to 10 μm and at a depth of 105 to 110μm were measured, and the difference in the normalized intensities at adepth of 0 to 10 μm between the bottom surface (surface B) and the topsurface (surface T) was calculated. In Table 1, “H/Si” indicates a valueobtained by dividing the average value at a depth of 0 to 10 μm by theaverage value at a depth of 105 to 110 μm.

Here, field aperture of detector was 1, and ESA input lens of detectorwas 550.

(2) Measurement of Warpage Amount

After measuring the warpage amount by a contact-type surface profileanalyzer (SURFCOM 1400D (trade name)) manufactured by Tokyo SeimitsuCo., Ltd. before chemical strengthening, each float glass was chemicallystrengthened with potassium nitrate molten salt at 425° C. for 150minutes. The warpage amount after chemical strengthening was measured inthe same manner, and the Δwarpage amount represented by (formula)Δwarpage amount=warpage amount after chemical strengthening−warpageamount before chemical strengthening was calculated. Here, the Δwarpageamount was the Δwarpage amount measured in a 9 cm-square float glass.

(3) Measurements of Na₂O Concentration and K₂O Concentration

As for the Na₂O concentration and K₂O concentration in the glass surfacelayer, the intensities of Na-Kα ray and K-Kα ray were measuredrespectively by fluorescent X-ray analysis by using ZSX Primus IImanufactured by Rigaku Corporation, and the concentration was determinedfrom the relative intensity ratio to the standard sample.

In Table 1, “N—Na₂O” is the normalized Na₂O surface concentration thatis a value obtained by dividing the Na₂O concentration by the Na₂Oconcentration at a depth position of 100 μm. Here, the Na₂Oconcentration is a value calculated from the relative intensity ratio toa standard sample by measuring the intensity of Na-Kα ray by fluorescentX-ray analysis. The ion exchange amount (K₂O concentration) afterchemical strengthening is shown in the column of K₂O Ion Exchange Amountin Table 1.

(4) Measurement of Sn Concentration

Regarding the Sn concentration in the glass surface, the glass surfacewas etched with a hydrofluoric acid solution and the Sn concentration inthe solution was quantitatively determined by ICP emission spectrometry.As for the ICP emission spectrometry, SPS3100 manufactured by SIINanoTechnology Inc. was used.

In Table 1, the glass of Compositions A to C does not contain SnO₂, andthe SnO₂ concentration in the top surface was evidently 0 and therefore,was not measured. The same applies to Tables 2 to 6 below.

With respect to the glass of Examples 1 to 4, the normalized hydrogenconcentration [0-10 μm average (H/Si)/105-110 μm (average H/Si)], N—Na₂Oconcentration (surface concentration/concentration at depth position of100 μm; hereinafter, also referred to as normalized Na₂O surfaceconcentration), K₂O concentration and Sn concentration (depositionamount per unit area), before chemical strengthening, and the ionexchange amount (K₂O concentration) and Δwarpage amount, after chemicalstrengthening, were determined, and the results are shown in Table 1.

The reason why the N—Na₂O concentration in the top surface is less than1 is considered because the SO₂ gas sprayed onto the bottom surfaceflowed around the top surface side. In addition, the reason why theN—Na₂O concentration in the top surface differs, for example, betweenExample 1 and Example 4 is considered because the SO₂ gas spraying statefluctuated.

TABLE 1 Measurement Results After Chemical Strengthening MeasurementResults Before K₂O Ion Com- Ex- Chemical Strengthening Exchange ΔWarpageposi- am- K₂O SnO₂ Amount Amount tion ple Surface H/Si N—Na₂O (wt %)(μg/cm²) (wt %) (μm) A 1 Top 1.02 0.94 0.327 — 5.01 58 Bottom 1.35 0.910.323 6.18 4.71 B 2 Top 1.53 0.93 0.310 — 4.93 52 Bottom 1.60 0.91 0.3055.46 4.58 3 Top 1.54 0.93 0.305 — 4.89 60 Bottom 1.91 0.90 0.300 5.654.53 4 Top 1.23 0.92 0.269 — 5.39 57 Bottom 1.54 0.90 0.265 6.06 5.04 5Top 1.27 0.92 0.268 — 5.35 69 Bottom 2.02 0.89 0.265 6.13 4.87 6 Top1.24 0.92 0.282 — 5.35 56 Bottom 1.64 0.91 0.279 5.57 5.06 C 7 Top 1.100.94 0.275 — 5.03 61 Bottom 1.38 0.91 0.272 6.05 4.74

Practical Example 2 Na₂O Concentration Difference in Top Surface andBottom Surface Before Chemical Strengthening and ΔWarpage Amount

Since the Na₂O concentration difference between the top surface and thebottom surface of a soda-lime glass sheet before chemical strengtheningis considered to have a correlation with the Δwarpage amount, thedifference Δ(N—Na₂O²) between squared normalized Na₂O surfaceconcentrations in the top surface and the bottom surface was determinedfrom the data shown in Table 1, and the correlation with the Δwarpageamount was studied.

The results are shown in Table 2 and FIG. 6. FIG. 6 is a graph plotting,on the abscissa, the difference Δ(N—Na₂O²) (Top-Bottom) between squarednormalized Na₂O surface concentrations in the top surface and the bottomsurface of glass to be subjected to chemical strengthening, and on theordinate, Δwarpage amount.

From the graph shown in FIG. 6, it was found that the differenceΔ(N—Na₂O²) between squared normalized Na₂O surface concentrations in thetop surface and the bottom surface of a soda-lime glass sheet beforechemical strengthening and the Δwarpage amount have a correlationrepresented by the following formula (1-1).

ΔWarpage amount=370×Δ(N—Na₂O²)+45  formula (1-1)

In formula (1-1), the Δ(N—Na₂O²) is the difference between squares ofthe values obtained by measuring, by fluorescent X-ray analysis, thenormalized Na₂O surface concentration in the top surface and the bottomsurface of glass to be subjected to chemical strengthening and isdetermined according to the following formula (1-2)

ΔNa₂O=(normalized Na₂O surface concentration in top surface beforechemical strengthening)²−(normalized Na₂O surface concentration inbottom surface before chemical strengthening)²  formula (1-2)

It was understood from the results shown in Table 2 and FIG. 6 that whenΔ(N—Na₂O²) in formula (1-1) is 0.040 or less, the Δwarpage amount can be58 μm or less.

TABLE 2 Δ(N—Na₂O{circumflex over ( )}2) ΔWarpage Ex- (Top − Amount ampleSurface N—Na₂O N—Na₂O{circumflex over ( )}2 Bottom) (μm) 1 Top 0.940.881 0.045 58 Bottom 0.91 0.836 2 Top 0.93 0.865 0.028 52 Bottom 0.910.837 3 Top 0.93 0.859 0.042 60 Bottom 0.90 0.817 4 Top 0.92 0.847 0.02857 Bottom 0.90 0.819 5 Top 0.92 0.838 0.046 69 Bottom 0.89 0.791 6 Top0.92 0.852 0.024 56 Bottom 0.91 0.828 7 Top 0.94 0.877 0.049 61 Bottom0.91 0.828

Practical Example 3 Ion Exchange Amount Difference Between Top Surfaceand Bottom Surface after Chemical Strengthening and ΔWarpage Amount

Since the ion exchange amount difference between the top surface and thebottom surface is considered to have a correlation with the Δwarpageamount, the ion exchange amount difference (Δion exchange amount 1) wasdetermined from the data shown in Table 1, and the correlation with theΔwarpage amount was studied.

The Δion exchange amount 1 is a value obtained by subtracting the ionexchange amount 1 in the bottom surface from the ion exchange amount 1in the top surface. The ion exchange amount 1 was determined accordingto the following formula (2-1).

Ion exchange amount 1=5.51×(normalized Na₂O surfaceconcentration)−0.038×(Sn concentration)  formula (2-1)

Ion exchange amount 1 difference=(ion exchange amount in topsurface)−(ion exchange amount in bottom surface)  formula (2-2)

The results are shown in Table 3 and FIG. 7. FIG. 7 is a graph plotting,on the abscissa, the Δion exchange amount 1 that is the difference inthe ion exchange amount 1 between the top surface and the bottomsurface, and on the ordinate, the Δwarpage amount.

From the graph shown in FIG. 7, it was found that the Δion exchangeamount 1 and the Δwarpage amount have a correlation represented by thefollowing formula (2-3).

ΔWarpage amount=103×(Δion exchange amount 1)+24  formula (2-3)

It was understood from the results shown in Table 3 and FIG. 7 that whenΔion exchange amount 1 is 0.32 or less, the Δwarpage amount can be 58 μmor less.

TABLE 3 Ex- Ion ΔIon ΔWarpage am- SnO₂ Exchange Exchange Amount pleSurface N—Na₂O (μg/cm²) Amount Amount (μm) 1 Top 0.94 — 5.18 0.37 58Bottom 0.91 6.18 4.81 2 Top 0.93 — 5.13 0.29 52 Bottom 0.91 5.46 4.83 3Top 0.93 — 5.11 0.34 60 Bottom 0.90 5.65 4.77 4 Top 0.92 — 5.07 0.32 57Bottom 0.90 6.06 4.76 5 Top 0.92 — 5.05 0.38 69 Bottom 0.89 6.13 4.67 6Top 0.92 — 5.09 0.28 56 Bottom 0.91 5.57 4.80 7 Top 0.94 — 5.16 0.38 61Bottom 0.91 6.05 4.79

Practical Example 4 Hydrogen Concentration Difference and SnConcentration Difference Between Top Surface and Bottom Surface BeforeChemical Strengthening, Ion Exchange Amount Difference, and ΔWarpageAmount

Since a correlation is considered to exist among the hydrogenconcentration difference and Sn concentration difference between the topsurface and the bottom surface before chemical strengthening, the ionexchange amount difference and the Δwarpage amount, multiple regressionanalysis was performed based on the data shown in Table 1 by using thesevalues as factors and, as a result, the following formula (3-1) wasobtained. The data obtained from the data shown in Table 1 is shown inTable 4.

W1=−16×(ΔH/Si)−6.47×(Sn concentration difference)−43.8×(Δion exchangeamount 1)  formula (3-1)

In formula (3-1), ΔH/Si is the difference (difference in the normalizedhydrogen concentration) between values obtained by measuring, by SIMSanalysis, the hydrogen concentration difference between the top surfaceand the bottom surface before chemical strengthening and is determinedaccording to the following formula (3-2).

ΔH/Si=(normalized hydrogen concentration in top surface before chemicalstrengthening)−(normalized hydrogen concentration in bottom surfacebefore chemical strengthening)  formula (3-2)

The Δion exchange amount 1 is a value obtained by subtracting the ionexchange amount in the bottom surface from the ion exchange amount inthe top surface. The ion exchange amount is determined according to theabove-mentioned formula (2-1).

FIG. 12 shows a graph plotting W1 on the abscissa and Δwarpage amount onthe ordinate. The graph shown in FIG. 12 revealed that W1 has acorrelation with the Δwarpage amount. It was understood from the resultsshown in Table 4 and FIG. 12 that when W1 is 56 or less, the Δwarpageamount can be 58 μm or less.

TABLE 4 Ex- Ion ΔIon ΔWarpage am- SnO₂ Exchange Exchange W1 Amount pleSurface H/Si Δ(H/Si) N—Na₂O (μg/cm²) Amount Amount (μm) (μm) 1 Top 1.02−0.33 0.94 — 5.18 0.37 61 58 Bottom 1.35 0.91 6.18 4.81 2 Top 1.53 −0.080.93 — 5.13 0.29 49 52 Bottom 1.60 0.91 5.46 4.83 3 Top 1.54 −0.37 0.93— 5.11 0.34 57 60 Bottom 1.91 0.90 5.65 4.77 4 Top 1.23 −0.31 0.92 —5.07 0.32 58 57 Bottom 1.54 0.90 6.06 4.76 5 Top 1.27 −0.75 0.92 — 5.050.38 68 69 Bottom 2.02 0.89 6.13 4.67 6 Top 1.24 −0.40 0.92 — 5.09 0.2855 56 Bottom 1.64 0.91 5.57 4.80 7 Top 1.10 −0.28 0.94 — 5.16 0.38 60 61Bottom 1.38 0.91 6.05 4.79

Practical Example 5 Ion Exchange Amount Difference Between Top Surfaceand Bottom Surface, Hydrogen Concentration Difference Between TopSurface and Bottom Surface Before Chemical Strengthening and ΔWarpageAmount

Since a correlation is considered to exist among the ion exchange amountdifference between the top surface and the bottom surface, the hydrogenconcentration difference between the top surface and the bottom surfacebefore chemical strengthening and the Δwarpage amount, the correlationamong the ion exchange amount difference between the top surface and thebottom surface, the hydrogen concentration difference between the topsurface and the bottom surface before chemical strengthening, and theΔwarpage amount was studied from the data shown in Table 1.

The results are shown in Table 5 and FIG. 8. FIG. 8 is a graph plotting,on the abscissa, a value obtained by dividing the ion exchange amountdifference between the top surface and the bottom surface by thehydrogen concentration difference between the top surface and the bottomsurface before chemical strengthening, and on the ordinate, the Δwarpageamount.

In addition, since a correlation is considered to exist among thehydrogen concentration, the ion exchange amount and the Δwarpage amount,multiple regression analysis was performed based on the data shown inTable 1 by using these values as factors and, as a result, the followingformula (4-1) was obtained.

W2=9.18×Δ[(ion exchange amount)/(H/Si)]+49  formula (4-1)

In formula (4-1), the Δ[(ion exchange amount)/(H/Si)] is a valuedetermined by subtracting a value obtained by dividing the ion exchangeamount in the bottom surface by the normalized hydrogen concentrationH/Si, from a value obtained by dividing the ion exchange amount in thetop surface by the normalized hydrogen concentration H/Si. The ionexchange amount was determined according to the above formula (2-1).

FIG. 13 shows a graph plotting W2 on the abscissa and Δwarpage amount onthe ordinate. The graph shown in FIG. 13 revealed that W2 has acorrelation with the Δwarpage amount. It was understood from the resultsshown in Table 5 and FIG. 13 that when W2 is 56 or less, the Δwarpageamount can be 58 μm or less.

TABLE 5 Ion Ion ΔIon Ex- Ex- Exchange Exchange ΔWarpage am- changeAmount/ Amount W2 Amount ple Surface H/Si Amount (H/Si) (H/Si) (μm) (μm)1 Top 1.02 5.18 5.08 1.51 63 58 Bottom 1.35 4.81 3.56 2 Top 1.53 5.133.36 0.35 52 52 Bottom 1.60 4.83 3.01 3 Top 1.54 5.11 3.32 0.82 57 60Bottom 1.91 4.77 2.50 4 Top 1.23 5.07 4.11 1.03 58 57 Bottom 1.54 4.763.09 5 Top 1.27 5.05 3.98 1.67 64 69 Bottom 2.02 4.67 2.31 6 Top 1.245.09 4.10 1.17 60 56 Bottom 1.64 4.80 2.92 7 Top 1.10 5.16 4.69 1.21 6061 Bottom 1.38 4.79 3.48

Practical Example 6 Na₂O Concentration Difference and Sn ConcentrationDifference Between Top Surface and Bottom Surface Before ChemicalStrengthening, and ΔWarpage Amount

Since the normalized Na₂O surface concentration difference and Snconcentration difference between the top surface and the bottom surfacebefore chemical strengthening are considered to have a correlation withthe Δwarpage amount, multiple regression analysis was performed based onthe data shown in Table 1 by using these values as factors. The resultsare shown in Table 6, FIG. 9 and FIG. 17.

As shown in FIG. 17, it was found that the normalized Na₂O surfaceconcentration difference and Sn concentration difference between the topsurface and the bottom surface before chemical strengthening have acorrelation represented by the following formula (5-1).

W3=744×[(ΔN—Na₂O)+0.01×(Sn concentration difference)]  formula (5-1)

In formula (5-1), the ΔN—Na₂O is a value obtained by subtracting thenormalized Na₂O surface concentration in the bottom surface from thenormalized Na₂O surface concentration in the top surface and is obtainedaccording to the following formula (5-2).

ΔN—Na₂O=(normalized Na₂O surface concentration in top surface beforechemical strengthening)−(normalized Na₂O surface concentration in bottomsurface before chemical strengthening)  formula (5-2)

FIG. 14 shows a graph plotting W3 on the abscissa and Δwarpage amount onthe ordinate. The graph shown in FIG. 14 revealed that W3 has acorrelation with the Δwarpage amount. It was understood from the resultsshown in Table 6 and FIG. 14 that when W3 is 58 or less, the Δwarpageamount can be 58 μm or less.

TABLE 6 ΔWarpage SnO₂ ΔN—Na₂O + Amount Example Surface N—Na₂O ΔN—Na₂O(μg/cm²) 0.01*SnO₂ W3 (μm) (μm) 1 Top 0.94 0.024 6.18 0.086 64 58 Bottom0.91 2 Top 0.93 0.015 5.46 0.070 52 52 Bottom 0.91 3 Top 0.93 0.023 5.650.080 59 60 Bottom 0.90 4 Top 0.92 0.015 6.06 0.076 57 57 Bottom 0.90 5Top 0.92 0.026 6.13 0.087 65 69 Bottom 0.89 6 Top 0.92 0.013 5.57 0.06951 56 Bottom 0.91 7 Top 0.94 0.026 6.05 0.087 65 61 Bottom 0.91

Practical Example 7 Ion Exchange Amount (Hydrogen Concentration, Na₂OConcentration and Sn Concentration) Difference and ΔWarpage Amount

Since the hydrogen concentration, Na₂O concentration and Snconcentration are considered to be responsible for the ion exchangeamount, a correlation formula was determined from the data shown inTable 7 and, as a result, the following formula (6-1) was obtained.

Ion exchange amount 2=−0.02×(H/Si)+5.54×(N—Na₂O concentration)−0.037×(Snconcentration)  formula (6-1)

Since the ion exchange amount difference between the top surface and thebottom surface is considered to affect the Δion exchange amount 2determined according to formula (6-1), i.e., the difference in the ionexchange amounts 2 between the top surface and the bottom surface, andthe warpage of glass after chemical strengthening, the correlation ofthe Δion exchange amount 2 determined according to formula (6-2), i.e.,the difference in the ion exchange amounts 2 between the top surface andthe bottom surface, with the Δwarpage amount was examined.

ΔIon exchange amount 2=(ion exchange amount 2 in top surface)−(ionexchange amount 2 in bottom surface)  formula (6-2)

FIG. 15 shows a graph plotting, on the abscissa, the difference (Δionexchange amount 2) between ion exchange amounts 2 in top surface andbottom surface, and on the ordinate, Δwarpage amount. The graph shown inFIG. 15 revealed that the Δion exchange amount 2 and the Δwarpage amounthave a correlation.

It was understood from the results shown in Table 7 and FIG. 15 thatwhen Δion exchange amount 2 in formula (6-1) is 0.33 or less, theΔwarpage amount can be 58 μm or less.

TABLE 7 Com- Ion ΔIon ΔWarpage posi- Ex- SnO₂ Exchange Exchange Amounttion ample Surface H/Si N—Na₂O (μg/cm²) Amount 2 Amount 2 (μm) A 1 Top1.02 0.94 0 5.18 0.37 58 Bottom 1.35 0.91 6.18 4.81 B 2 Top 1.53 0.93 05.12 0.29 52 Bottom 1.60 0.91 5.46 4.83 3 Top 1.54 0.93 0 5.10 0.34 60Bottom 1.91 0.90 5.65 4.76 4 Top 1.23 0.92 0 5.07 0.32 57 Bottom 1.540.90 6.06 4.76 5 Top 1.27 0.92 0 5.05 0.38 69 Bottom 2.02 0.89 6.13 4.666 Top 1.24 0.92 0 5.09 0.29 56 Bottom 1.64 0.91 5.57 4.80 C 7 Top 1.100.94 0 5.17 0.38 61 Bottom 1.38 0.91 6.05 4.79

Example 7 Na₂O Concentration Difference Between Top Surface and BottomSurface Before Chemical Strengthening and ΔWarpage Amount

Since the Na₂O concentration difference between the top surface and thebottom surface of a soda-lime glass sheet before chemical strengtheningis considered to have a correlation with the Δwarpage amount, a square[ΔN—Na₂O (Top-Bottom)]² of the difference obtained by subtracting thenormalized Na₂O surface concentration of the bottom surface from thenormalized Na2O surface concentration of the top surface was determinedfrom the data shown in Table 1, and the correlation with the Δwarpageamount was studied.

The results are shown in Table 8 and FIG. 16. In Table 8, for example,“5.9.E-04” means 5.9×10⁻⁴. FIG. 16 is a graph plotting, on the abscissa,a square [ΔN—Na₂O (Top-Bottom)]² of the difference obtained bysubtracting the normalized Na₂O surface concentration of the bottomsurface from the normalized Na₂O surface concentration of the topsurface to be subjected to chemical strengthening, and on the ordinate,Δwarpage amount.

The graph shown in FIG. 16 revealed that the square (ΔN—Na₂O)² of thedifference between normalized Na₂O surface concentrations in the topsurface and the bottom surface of a soda-lime glass sheet beforechemical strengthening and the Δwarpage amount have a correlationrepresented by the following formula (7-1).

ΔWarpage amount=18000×(ΔN—Na₂O)²+51  formula (7-1)

In formula (7-1), the (ΔN—Na₂O)² is a square of the difference betweenthe values obtained by measuring, by fluorescent X-ray analysis, thenormalized Na₂O surface concentration in the top surface and the bottomsurface of glass to be subjected to chemical strengthening and isdetermined according to the following formula (7-2):

(ΔN—Na₂O)²=[(normalized Na₂O surface concentration in top surface beforechemical strengthening)−(normalized Na₂O surface concentration in bottomsurface before chemical strengthening)]²  formula (7-2)

It was understood from the results shown in Table 8 and FIG. 16 thatwhen (ΔN—Na₂O)² in formula (7-1) is 5.0×10⁻⁴ or less, the Δwarpageamount can be 58 μm or less.

TABLE 8 ΔWarpage Example Surface N—Na₂O (ΔN—Na₂O){circumflex over ( )}2Amount (μm) 1 Top 0.94 5.9.E−04 58 Bottom 0.91 2 Top 0.93 2.3.E−04 52Bottom 0.91 3 Top 0.93 5.4.E−04 60 Bottom 0.90 4 Top 0.92 2.4.E−04 57Bottom 0.90 5 Top 0.92 6.6.E−04 69 Bottom 0.89 6 Top 0.92 1.7.E−04 56Bottom 0.91 7 Top 0.94 6.9.E−04 61 Bottom 0.91

Glass Composition Example

Composition Examples G1 to G16 in mass % of the float glass for chemicalstrengthening of the present invention, and the compressive stress CS(unit: MPa) and depth of compressive stress layer DOL (unit: μm) whenthe float glass was chemically strengthened, are shown in Tables 9 and10.

In the Tables, Na₂O/Al₂O₃ is the ratio of Na₂O and Al₂O₃ contents, RO isthe total of MgO, CaO, SrO and BaO contents, CaO+SrO+BaO is the total ofCaO, SrO and BaO contents, strengthening temperature (unit: ° C.) andstrengthening time (unit: h) are those in the chemical strengtheningabove, KNO₃ is the concentration (unit: mass %) of KNO₃ in a molten saltused for chemical strengthening, and dol is the dol described above.Here, when the concentration of KNO₃ in a molten salt is not 100%, theremaining component is NaNO₃.

TABLE 9 G1 G2 G3 G4 G5 G6 G7 G8 G9 SiO₂ 72 72 72 67 62 60 68 65.5 69Al₂O₃ 2 2 2 1 1 1 5 3 2 MgO 4.5 4.5 4.5 6 8 10 5 6 4 CaO 8 8 8 9 10 13 78 10 SrO 0 0 0 0 0 0 1 0 0 BaO 0 0 0 0 0 0 0 0 0 ZrO₂ 0 0 0 0 0 0 0 2.51 Na₂O 13.2 13.2 13.2 15 14 10 13 9 12 K₂O 0.3 0.3 0.3 2 5 6 1 6 2 Na₂O/6.6 6.6 6.6 15.0 14.0 10.0 2.6 3.0 6.0 Al₂O₃ RO 12.5 12.5 12.5 15.0 18.023.0 13.0 14.0 14.0 CaO + 8.0 8.0 8.0 9.0 10.0 13.0 8.0 8.0 10.0 SrO +BaO Strength- 425 425 425 425 425 450 425 425 425 ening temper- atureStrength- 2.5 2.5 2.5 2.5 2.5 5 2.5 2.5 4 ening time t KNO₃ 100 97.5 95100 100 100 100 100 100 dol 5 5 5 7 6 4 5 5 6 CS 680 580 480 690 730 810760 770 710 DOL 5 5 5 7 6 4 5 5 6

TABLE 10 G10 G11 G12 G13 G14 G15 G16 G17 G18 SiO₂ 65 62.5 65 76.5 77.971 74 72 69 Al₂O₃ 5 7.5 7 0.5 0.1 4 4 6 5 MgO 6 3 4 3 2 4 3.5 4 3 CaO 56 2 5 4 7 1.5 1 1 SrO 2 0 1 0 0 0 0 2 0 BaO 0 0 1 0 0 0 0 0 2 ZrO₂ 0.2 12 0 4 0 0 0 0 Na₂O 14.3 20 16.5 14 12 10 15 11 17 K₂O 2.5 0 1.5 1 0 4 24 3 Na₂O/ 2.9 2.7 2.4 28.0 120 2.5 3.8 1.8 3.4 Al₂O₃ RO 13.0 9.0 8.0 8.06.0 11.0 5.0 7.0 6.0 CaO + 7.0 6.0 4.0 5.0 4.0 7.0 1.5 3.0 3.0 SrO + BaOStrength- 425 425 425 425 425 425 400 400 400 ening temper- atureStrength- 2.5 2.5 1 2.5 2.5 2.5 1 1 0.5 ening time t KNO₃ 100 100 100100 100 100 100 100 100 dol 12 20 16 18 10 12 17 12 19 CS 720 720 730470 590 630 550 630 560 DOL 12 20 16 18 10 12 17 12 19

While the present invention has been described in detail with referenceto specific embodiments, it is apparent to one skilled in the art thatvarious changes and modifications can be made therein without departingfrom the intension and scope of the present invention. This applicationis based on Japanese Patent Application (No. 2012-285511) filed on Dec.27, 2012, and the whole of which are incorporated by reference.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

-   -   1 Molten glass    -   5 Molten metal bath    -   10 Display device    -   15 Housing    -   20 Display panel    -   30 Cover glass

1. A float glass for chemical strengthening, comprising a bottom surfacecoming into contact with a molten metal at the time of forming and a topsurface opposing said bottom surface, wherein a difference Δ(N—Na₂O²)determined by subtracting a square of a normalized Na₂O surfaceconcentration of the bottom surface which is a value obtained bydividing an Na₂O concentration in said bottom surface by an Na₂Oconcentration at a depth position of 100 μm therefrom, from a square ofa normalized Na₂O surface concentration of the top surface which is avalue obtained by dividing an Na₂O concentration in said top surface byan Na₂O concentration at a depth position of 100 μm therefrom, is 0.040or less, where each Na₂O concentration is a value measured by afluorescent X-ray analysis using an Na-Kα ray.
 2. A float glass forchemical strengthening, comprising a bottom surface coming into contactwith a molten metal at the time of forming and a top surface opposingsaid bottom surface, wherein a Δion exchange amount 1 which is a valueobtained by subtracting an ion exchange amount 1 in said bottom surfacefrom an ion exchange amount 1 in said top surface is 0.32 or less, wherethe ion exchange amount 1 is a value determined according to thefollowing formula (2-1):Ion exchange amount 1=5.51×(normalized Na₂O surfaceconcentration)−0.038×(Sn concentration)  formula (2-1) in formula (2-1),the normalized Na₂O surface concentration is a value obtained bydividing an Na₂O concentration at the surface by an Na₂O concentrationat a depth position of 100 μm, where the Na₂O concentration is a valuemeasured by a fluorescent X-ray analysis using an Na-Kα ray; and the Snconcentration is an Sn deposition amount (unit: μg/cm²) per unit area ofthe top surface and the bottom surface, where the Sn deposition amountper unit area is a deposition mass in terms of SnO₂ per 1 cm² when Sn isassumed to exist in the form of SnO₂.
 3. A float glass for chemicalstrengthening, comprising a bottom surface coming into contact with amolten metal at the time of forming and a top surface opposing saidbottom surface, wherein W1 determined according to the following formula(3-1) is 56 or less:W1=−16×(ΔH/Si)−6.47×(Sn concentration difference)−43.8×(Δion exchangeamount 1)  formula (3-1) in formula (3-1), the ΔH/Si is a value obtainedby subtracting a normalized hydrogen concentration in the bottom surfacefrom a normalized hydrogen concentration in the top surface, where thenormalized hydrogen concentration is a value obtained by dividing anaverage hydrogen concentration at a depth of 0 to 10 μm by an averagehydrogen concentration at a depth of 105 to 110 μm, wherein the averagehydrogen concentration at a depth of 0 to 10 μm and the average hydrogenconcentration at a depth of 105 to 110 μm are values measured under thefollowing analysis conditions: (Analysis Conditions) Measurementapparatus: secondary ion mass spectrometry apparatus with a quadrupolemass spectrometer Primary ion species: Cs⁺ Primary accelerating voltage:5.0 kV Primary ion current: 1 μA Primary ion incident angle (angle fromdirection perpendicular to sample surface): 60° Luster size: 200×200 μm²Detection region: 40×40 μm² Secondary ion polarity: minus Electron gunfor neutralization: used; in formula (3-1), the Sn concentrationdifference is a difference obtained by subtracting an Sn depositionamount (unit: μg/cm²) per unit area of the top surface from an Sndeposition amount (unit: μg/cm²) per unit area of the bottom surface,where the Sn deposition amount per unit area is a deposition mass interms of SnO₂ per 1 cm² when Sn is assumed to exist in the form of SnO₂;and in formula (3-1), the Δion exchange amount 1 is a value obtained bysubtracting an ion exchange amount 1 in said bottom surface from an ionexchange amount 1 in said top surface, where the ion exchange amount 1is determined according to the following formula:Ion exchange amount 1=5.51×(normalized Na₂O surfaceconcentration)−0.038×(Sn concentration) wherein the normalized Na₂Osurface concentration is a value obtained by dividing an Na₂Oconcentration at the surface by an Na₂O concentration at a depthposition of 100 μm, where the Na₂O concentration is a value measured bya fluorescent X-ray analysis using an Na-Kα ray.
 4. A float glass forchemical strengthening, comprising a bottom surface coming into contactwith a molten metal at the time of forming and a top surface opposingsaid bottom surface, wherein an absolute value of W2 determinedaccording to the following formula (4-1) is 56 or less:W2=9.18×Δ[(ion exchange amount 1)/(H/Si)]+49  formula (4-1) in formula(4-1), the Δ[(ion exchange amount 1)/(H/Si)] is a value determined bysubtracting a value obtained by dividing an ion exchange amount 1 in thebottom surface by a normalized hydrogen concentration H/Si in the samesurface, from a value obtained by dividing an ion exchange amount 1 inthe top surface by a normalized hydrogen concentration H/Si in the samesurface, where the ion exchange amount 1 is determined according to thefollowing formula:Ion exchange amount 1=5.51×(normalized Na₂O surfaceconcentration)−0.038×(Sn concentration) in the formula, the normalizedNa₂O surface concentration is a value obtained by dividing an Na₂Oconcentration at the surface by an Na₂O concentration at a depthposition of 100 μm, where the Na₂O concentration is a value measured bya fluorescent X-ray analysis using an Na-Kα ray, and the Snconcentration is an Sn deposition amount (unit: μg/cm²) per unit area ofthe top surface and the bottom surface, where the Sn deposition amountper unit area is a deposition mass in terms of SnO₂ per 1 cm² when Sn isassumed to exist in the form of SnO₂; and where the normalized hydrogenconcentration is a value obtained by dividing an average hydrogenconcentration at a depth of 0 to 10 μm by an average hydrogenconcentration at a depth of 105 to 110 μm, wherein the average hydrogenconcentration at a depth of 0 to 10 μm and the average hydrogenconcentration at a depth of 105 to 110 μm are values measured under thefollowing analysis conditions: (Analysis Conditions) Measurementapparatus: secondary ion mass spectrometry apparatus with a quadrupolemass spectrometer Primary ion species: Cs⁺ Primary accelerating voltage:5.0 kV Primary ion current: 1 μA Primary ion incident angle (angle fromdirection perpendicular to sample surface): 60° Luster size: 200×200 μm²Detection region: 40×40 μm² Secondary ion polarity: minus Electron gunfor neutralization: used.
 5. The float glass for chemical strengtheningas claimed in claim 1, comprising a bottom surface coming into contactwith a molten metal at the time of forming and a top surface opposingsaid bottom surface, wherein W3 determined according to the followingformula (5-1) is 58 or less:W3=744×[(ΔN—Na₂O)+0.01×(Sn concentration difference)]  formula (5-1) informula (5-1), the ΔN—Na₂O is a value determined by subtracting anormalized Na₂O surface concentration of the bottom surface which is avalue obtained by dividing an Na₂O concentration at the surface in thebottom surface by an Na₂O concentration at a depth position of 100 μmtherefrom, from the normalized Na₂O surface concentration of the topsurface which is a value obtained by dividing an Na₂O concentration atthe surface in the top surface by an Na₂O concentration at a depthposition of 100 μm therefrom, where each Na₂O concentration is a valuemeasured by a fluorescent X-ray analysis using an Na-Kα ray; and informula (5-1), the Sn concentration difference is a difference obtainedby subtracting an Sn deposition amount (unit: μg/cm²) per unit area ofthe top surface from an Sn deposition amount (unit: μg/cm²) per unitarea of the bottom surface, where the Sn deposition amount per unit areais a deposition mass in terms of SnO₂ per 1 cm² when Sn is assumed toexist in the form of SnO₂.
 6. The float glass for chemical strengtheningas claimed in claim 2, comprising a bottom surface coming into contactwith a molten metal at the time of forming and a top surface opposingsaid bottom surface, wherein W3 determined according to the followingformula (5-1) is 58 or less:W3=744×[(ΔN—Na₂O)+0.01×(Sn concentration difference)]  formula (5-1) informula (5-1), the ΔN—Na₂O is a value determined by subtracting anormalized Na₂O surface concentration of the bottom surface which is avalue obtained by dividing an Na₂O concentration at the surface in thebottom surface by an Na₂O concentration at a depth position of 100 μmtherefrom, from the normalized Na₂O surface concentration of the topsurface which is a value obtained by dividing an Na₂O concentration atthe surface in the top surface by an Na₂O concentration at a depthposition of 100 μM therefrom, where each Na₂O concentration is a valuemeasured by a fluorescent X-ray analysis using an Na-Kα ray; and informula (5-1), the Sn concentration difference is a difference obtainedby subtracting an Sn deposition amount (unit: μg/cm²) per unit area ofthe top surface from an Sn deposition amount (unit: μg/cm²) per unitarea of the bottom surface, where the Sn deposition amount per unit areais a deposition mass in terms of SnO₂ per 1 cm² when Sn is assumed toexist in the form of SnO₂.
 7. The float glass for chemical strengtheningas claimed in claim 3, comprising a bottom surface coming into contactwith a molten metal at the time of forming and a top surface opposingsaid bottom surface, wherein W3 determined according to the followingformula (5-1) is 58 or less:W3=744×[(ΔN—Na₂O)+0.01×(Sn concentration difference)]  formula (5-1) informula (5-1), the ΔN—Na₂O is a value determined by subtracting anormalized Na₂O surface concentration of the bottom surface which is avalue obtained by dividing an Na₂O concentration at the surface in thebottom surface by an Na₂O concentration at a depth position of 100 μmtherefrom, from the normalized Na₂O surface concentration of the topsurface which is a value obtained by dividing an Na₂O concentration atthe surface in the top surface by an Na₂O concentration at a depthposition of 100 μm therefrom, where each Na₂O concentration is a valuemeasured by a fluorescent X-ray analysis using an Na-Kα ray; and informula (5-1), the Sn concentration difference is a difference obtainedby subtracting an Sn deposition amount (unit: μg/cm²) per unit area ofthe top surface from an Sn deposition amount (unit: μg/cm²) per unitarea of the bottom surface, where the Sn deposition amount per unit areais a deposition mass in terms of SnO₂ per 1 cm² when Sn is assumed toexist in the form of SnO₂.
 8. The float glass for chemical strengtheningas claimed in claim 4, comprising a bottom surface coming into contactwith a molten metal at the time of forming and a top surface opposingsaid bottom surface, wherein W3 determined according to the followingformula (5-1) is 58 or less:W3=744×[(ΔN—Na₂O)+0.01×(Sn concentration difference)]  formula (5-1) informula (5-1), the ΔN—Na₂O is a value determined by subtracting anormalized Na₂O surface concentration of the bottom surface which is avalue obtained by dividing an Na₂O concentration at the surface in thebottom surface by an Na₂O concentration at a depth position of 100 μmtherefrom, from the normalized Na₂O surface concentration of the topsurface which is a value obtained by dividing an Na₂O concentration atthe surface in the top surface by an Na₂O concentration at a depthposition of 100 μm therefrom, where each Na₂O concentration is a valuemeasured by a fluorescent X-ray analysis using an Na-Kα ray; and informula (5-1), the Sn concentration difference is a difference obtainedby subtracting an Sn deposition amount (unit: μg/cm²) per unit area ofthe top surface from an Sn deposition amount (unit: μg/cm²) per unitarea of the bottom surface, where the Sn deposition amount per unit areais a deposition mass in terms of SnO₂ per 1 cm² when Sn is assumed toexist in the form of SnO₂.
 9. The float glass for chemical strengtheningas claimed in claim 1, comprising a bottom surface coming into contactwith a molten metal at the time of forming and a top surface opposingsaid bottom surface, wherein Δion exchange amount 2 which is a valueobtained by subtracting an ion exchange amount 2 in said bottom surfacefrom an ion exchange amount 2 in said top surface is 0.33 or less, wherethe ion exchange amount 2 is a value determined according to thefollowing formula (6-1):Ion exchange amount 2=−0.02×(H/Si)+5.54×(N—Na₂O concentration)−0.037×(Snconcentration)  formula (6-1) in formula (6-1), the H/Si is a normalizedhydrogen concentration, where the normalized hydrogen concentration is avalue obtained by dividing an average hydrogen concentration at a depthof 0 to 10 μm by an average hydrogen concentration at a depth of 105 to110 μm, wherein the average hydrogen concentration at a depth of 0 to 10μm and the average hydrogen concentration at a depth of 105 to 110 μmare values measured under the following analysis conditions: (AnalysisConditions) Measurement apparatus: secondary ion mass spectrometryapparatus with a quadrupole mass spectrometer Primary ion species: Cs⁺Primary accelerating voltage: 5.0 kV Primary ion current: 1 μA Primaryion incident angle (angle from direction perpendicular to samplesurface): 60° Luster size: 200×200 μm² Detection region: 40×40 μm²Secondary ion polarity: minus Electron gun for neutralization: used; andin formula (6-1), the N—Na₂O concentration is a normalized Na₂O surfaceconcentration which is a value obtained by dividing a surface Na₂Oconcentration by an Na₂O concentration at a depth position of 100 μm,where the Na₂O concentration is a value measured by a fluorescent X-rayanalysis using an Na-Kα ray, and the Sn concentration is an Sndeposition amount (unit: μg/cm²) per unit area, where the Sn depositionamount is a deposition mass in terms of SnO₂ when Sn is assumed to existin the form of SnO₂.
 10. The float glass for chemical strengthening asclaimed in claim 2, comprising a bottom surface coming into contact witha molten metal at the time of forming and a top surface opposing saidbottom surface, wherein Δion exchange amount 2 which is a value obtainedby subtracting an ion exchange amount 2 in said bottom surface from anion exchange amount 2 in said top surface is 0.33 or less, where the ionexchange amount 2 is a value determined according to the followingformula (6-1):Ion exchange amount 2=−0.02×(H/Si)+5.54×(N—Na₂O concentration)−0.037×(Snconcentration)  formula (6-1) in formula (6-1), the H/Si is a normalizedhydrogen concentration, where the normalized hydrogen concentration is avalue obtained by dividing an average hydrogen concentration at a depthof 0 to 10 μm by an average hydrogen concentration at a depth of 105 to110 μm, wherein the average hydrogen concentration at a depth of 0 to 10μm and the average hydrogen concentration at a depth of 105 to 110 μmare values measured under the following analysis conditions: (AnalysisConditions) Measurement apparatus: secondary ion mass spectrometryapparatus with a quadrupole mass spectrometer Primary ion species: Cs⁺Primary accelerating voltage: 5.0 kV Primary ion current: 1 μA Primaryion incident angle (angle from direction perpendicular to samplesurface): 60° Luster size: 200×200 μm² Detection region: 40×40 μm²Secondary ion polarity: minus Electron gun for neutralization: used; andin formula (6-1), the N—Na₂O concentration is a normalized Na₂O surfaceconcentration which is a value obtained by dividing a surface Na₂Oconcentration by an Na₂O concentration at a depth position of 100 μm,where the Na₂O concentration is a value measured by a fluorescent X-rayanalysis using an Na-Kα ray, and the Sn concentration is an Sndeposition amount (unit: μg/cm²) per unit area, where the Sn depositionamount is a deposition mass in terms of SnO₂ when Sn is assumed to existin the form of SnO₂.
 11. The float glass for chemical strengthening asclaimed in claim 3, comprising a bottom surface coming into contact witha molten metal at the time of forming and a top surface opposing saidbottom surface, wherein Δion exchange amount 2 which is a value obtainedby subtracting an ion exchange amount 2 in said bottom surface from anion exchange amount 2 in said top surface is 0.33 or less, where the ionexchange amount 2 is a value determined according to the followingformula (6-1):Ion exchange amount 2=−0.02×(H/Si)+5.54×(N—Na₂O concentration)−0.037×(Snconcentration)  formula (6-1) in formula (6-1), the H/Si is a normalizedhydrogen concentration, where the normalized hydrogen concentration is avalue obtained by dividing an average hydrogen concentration at a depthof 0 to 10 μm by an average hydrogen concentration at a depth of 105 to110 μm, wherein the average hydrogen concentration at a depth of 0 to 10μm and the average hydrogen concentration at a depth of 105 to 110 μmare values measured under the following analysis conditions: (AnalysisConditions) Measurement apparatus: secondary ion mass spectrometryapparatus with a quadrupole mass spectrometer Primary ion species: Cs⁺Primary accelerating voltage: 5.0 kV Primary ion current: 1 μA Primaryion incident angle (angle from direction perpendicular to samplesurface): 60° Luster size: 200×200 μm² Detection region: 40×40 μm²Secondary ion polarity: minus Electron gun for neutralization: used; andin formula (6-1), the N—Na₂O concentration is a normalized Na₂O surfaceconcentration which is a value obtained by dividing a surface Na₂Oconcentration by an Na₂O concentration at a depth position of 100 μm,where the Na₂O concentration is a value measured by a fluorescent X-rayanalysis using an Na-Kα ray, and the Sn concentration is an Sndeposition amount (unit: μg/cm²) per unit area, where the Sn depositionamount is a deposition mass in terms of SnO₂ when Sn is assumed to existin the form of SnO₂.
 12. The float glass for chemical strengthening asclaimed in claim 4, comprising a bottom surface coming into contact witha molten metal at the time of forming and a top surface opposing saidbottom surface, wherein Δion exchange amount 2 which is a value obtainedby subtracting an ion exchange amount 2 in said bottom surface from anion exchange amount 2 in said top surface is 0.33 or less, where the ionexchange amount 2 is a value determined according to the followingformula (6-1):Ion exchange amount 2=−0.02×(H/Si)+5.54×(N—Na₂O concentration)−0.037×(Snconcentration)  formula (6-1) in formula (6-1), the H/Si is a normalizedhydrogen concentration, where the normalized hydrogen concentration is avalue obtained by dividing an average hydrogen concentration at a depthof 0 to 10 μm by an average hydrogen concentration at a depth of 105 to110 μm, wherein the average hydrogen concentration at a depth of 0 to 10μm and the average hydrogen concentration at a depth of 105 to 110 μmare values measured under the following analysis conditions: (AnalysisConditions) Measurement apparatus: secondary ion mass spectrometryapparatus with a quadrupole mass spectrometer Primary ion species: Cs⁺Primary accelerating voltage: 5.0 kV Primary ion current: 1 μA Primaryion incident angle (angle from direction perpendicular to samplesurface): 60° Luster size: 200×200 μm² Detection region: 40×40 μm²Secondary ion polarity: minus Electron gun for neutralization: used; andin formula (6-1), the N—Na₂O concentration is a normalized Na₂O surfaceconcentration which is a value obtained by dividing a surface Na₂Oconcentration by an Na₂O concentration at a depth position of 100 μm,where the Na₂O concentration is a value measured by a fluorescent X-rayanalysis using an Na-Kα ray, and the Sn concentration is an Sndeposition amount (unit: μg/cm²) per unit area, where the Sn depositionamount is a deposition mass in terms of SnO₂ when Sn is assumed to existin the form of SnO₂.
 13. The float glass for chemical strengthening asclaimed in claim 1, which is used for chemical strengthening in which achemical strengthening temperature is T (unit: K) and a chemicalstrengthening time is t (unit: hours) and contains SiO₂, wherein a doldetermined according to the following formula by using respectivecontents in mass % of SiO₂, Al₂O₃, MgO, CaO, SrO, BaO, ZrO₂, Na₂O andK₂O is 20 or less:dol=−0.13×Al₂O₃−1.88×MgO−2.41×CaO−1.85×SrO−1.35×BaO−1.59×ZrO₂+1.50×Na₂O+2.42×K₂O−129359/T+9.28×t^(0.5)+182.88.14. The float glass for chemical strengthening as claimed in claim 1,which contains, in mass %, from 60 to 80% of SiO₂, from 0 to 8% ofAl₂O₃, from 8 to 22% of Na₂O, from 0 to 7% of K₂O, from 0 to 17% of MgO,from 0 to 22% of CaO, from 0 to 8% of SrO, from 0 to 8% of BaO, and from0 to 5% of ZrO₂.
 15. The float glass for chemical strengthening asclaimed in claim 1, comprising, in mass %, from 60 to 80% of SiO₂, from0.01 to 8% of Al₂O₃, from 8 to 22% of Na₂O, from 0 to 7% of K₂O and from0 to 5% of ZrO₂, wherein in the case of containing MgO, CaO, SrO or BaO,the total of the MgO, CaO, SrO and BaO contents is from 5 to 25%, and aratio Na₂O/Al₂O₃ of Na₂O and Al₂O₃ contents is 1.5 or more.
 16. Thefloat glass for chemical strengthening as claimed in claim 15, whereinthe Na₂O/Al₂O₃ is 6 or less.
 17. The float glass for chemicalstrengthening as claimed in claim 14, comprising CaO, SrO or BaO,wherein the total of the CaO, SrO and BaO contents is from 1 to 7%. 18.A method for producing a chemically strengthened glass having a depth ofcompressive stress layer of 20 μm or less, comprising chemicallystrengthening the glass for chemical strengthening claimed in claim 1.